【Objective】The aim of this study was to explore effects and mechanism of herb roots in natural state improving shear strength of unconfined compressed solum, so as to provide a scientific basis for calculating the capacity of herb roots to reinforce slopes and selecting proper species of grasses to grow on slopes.【Method】The experiment was carried out at the experimental farm of the Yunnan Agricultural University, China. In January 2016, a total of 40 PVC tubes, 51cm in length, 110 mm in diameter and 3.2 mm in thickness were all cut in half, and then the halves were bound together by pair with rubber bands. Upland red soil < 5 mm in particle size was packed into in these rubber band fixed tubes with the bottom sealed with plastic film up to 50 cm. The soil in the tubes was 28.31% in moisture content and 0.78 g.cm^-1 in dry density. The tubes were divided into four groups, 10 each. Three groups were sown with seeds of Setariaanceps Stapf ex Massey L., Dactylisglomerata L. and Medicago sativa L. 12 seeds each tube, separately, in May, and the other group left unplanted as control for comparison, Besides, the three species of grasses were planted, separately, in the field, 1 m2 each in plot area for determination of tensile strength of the grass roots. In October, out of each group, 7 tubes were picked randomly, placed in water for 24 h until they were fully saturated, and then removed out of water. The tubes were split off and the soil columns inside taken out. Shoots of the plants were cut off. The soil columns were cut into two, 25 cm each, in the middle with a hacksaw. From each half of the soil columns, a section of 20 cm in the middle was taken as test samples and the section was 10.36 cm in diameter. The samples were analyzed for saturation density and saturated water content and tested for unconfined shear strength on a SJ-1A type strain controlling triaxial apparatus (made in Nanjing Soil Instrument Factory, China). The test went on in line with the geotechnical test code (SL237-1999) of China. Before the test the pressure cell and pressure system was removed from the apparatus and then the sample was put on the platform of the triaxial compression system for pressure test with a shearing rate of 4.14 mm?min^-1. A dial gauge was used to monitor deformation of the soil column and of the dynamometric ring and record the process of ess-strain until collapse of the sample or the total axial strain reaching 20%. For soil samples that stood the pressure even after the total axial strain reached 20%, their shear strength should be the value that corresponded to the one when the strain reached 15%. After the compression test, the soil columns were separated along the failure surface, and then the roots appearing on the failure surface were counted and measured with an electronic calipers for diameter; Biomass of the roots in the sample was measured after the samples were oven dried. In October, roots of the three species of grasses growing in the field were dug up and measured with an electronic calipers for diameter and with a Shandu SN100 tension tester and a universal testing machine for tensile resistance of each root.【Result】 (1) The root systems of all the species of grasses enhanced the cohesive strength (△C) of the 0~25 cm soil layer by 4.75 kPa for Setariaanceps Stapf ex Massey L., by 4.40 kPa for Dactylisglomerata L. and by 1.39 kPa for Medicago sativa L., and that of the 25~50 cm soil layer by 3.10, 2.32 and 0.71 kPa, respectively; (2) △C (increment in tensile strength) was significantly related to root density (RD) and root area ratio (RAR) of Setariaanceps Stapf ex Massey L. and Dactylisglomerata L. roots in the failure surface and root content (Q) in the soil-root complex. The relationship between Q and △C was the highest. However, △C was not related with mean diameter of the roots (Tr,a). In the case of Medicago sativa L. △C had nothing to do with all the four root parameters.【Conclusion】 Setariaanceps Stapf ex Massey L. is the highest and Medicago sativa L. the lowest in the effect of enhancing shear strength of the soil. △C of Medicago sativa L. varies sharply, being higher in the upper half section than in the lower half section. As Setariaanceps Stapf ex Massey L. and Dactylisglomerata L. are herbs dominated with oblique root, among the three root parameters, RD, RAR and Q, Q is the best index for calculating △C, which makes it feasible to use the probability theory to calcuate stability of the unconfined compression test of samples collected from a number of sampling points on a vegetated slope.