Investigations were done of micromorphological features of the soils derived from loess, with a view to revealing diagnostic significance of these features and providing reliable micromorphological basis so as to define positions of these soils in the Chinese Soil Taxonomy. A soil profile at Chafangcun, Shaanxi Province, China was selected as object of the study. Soil samples were collected from the profile for analysis of relevant soil physical and chemical properties (including chroma, magnetic susceptibility, particle-size composition, total iron and free iron content, etc.) in laboratory. The soil was prepared into thin slices, which were put under a Leica -DMRX petrographic microscope for observation of soil micromorphology and the images were processes with the Nis-Elments BR 3.2 image analysis software for extraction of micromorphologic unit images and quantitative measurement of relevant parameters. Results show that the Chafangcun soil profile has an A-AB-Bt-BC-C profile structure, with a large volume of secondary illuvial clay accumulated in Bt Horizon (110～230 cm in depth), reaching to 15% ~ 30% in content and making the illuvial clay/residual clay ratio (K_i/r) varying between 4 and 9. The illuvial clay minerals exist mainly in the form of irregular lumps, and then in the form of clay cutan, mostly < 0.15mm in thickness. Compared with Bt Horizon, the upper eluvial horizon is apparently lower in secondary clay mineral (< 5%), dominated with residual clay mostly in the form of concentrate; and the lower parent material layer is much lower in secondary clay (generally > 3%), mostly in the form of very thin clay cutan, sparsely present on edges of the pores. In the upper eluvial horizon coarse particles (> 10 μm) are generally in the form of subangular - subround lumps, 30.0 μm in average particle size, moderate in content (10.4%), and unevenly distributed in the horizon,. In the parent material layer, coarse particles are in the form of apparent sub-angular – angular lumps, 32.0 μm in average particle size, 24.6% in content and very evenly distributed in the horizon. But in Bt Horizon (110～230cm in depth), they are in the form of round - subround lumps, 25.4μm or the smallest in average particle size, 4.6% or the lowest in content in the profile and sparsely distributed patches in the horizon. The differences are obvious in mean particle size and content of clay particles between Bt horizon, the upper leached layer and the lower parent material layer. Pores in A horizon are very complex in shape with jigsaw like boundary, varied in pore size, and very unevenly distributed, appearing on the whole like an equiaxe. In the parent material layer, pores are small in size and limited in range of verification and evenly distributed, appearing on the whole like an equiaxe. But, pores in Bt horizon are small in number but big in size of single pores, appearing in the form of elongated fractures, with an average aspect ratio between 10 to 30. The pores have smooth edges coated with clay cutan. The above-described micromorphological features indicate that the soil profile at Chafangcun is characterized by vigorous leaching with plenty of water, and large volumes of secondary clay was formed in soil forming process, releasing a certain amount of free irons. Both of them keep migrating into Bt horizon and accumulating therein. These micromorphological features can be used as micromorphological index of the diagnostic horizon, “Argic horizon”, which reliably demonstrates that the Bt horizon (110～23 cm in depth) of the Chafangcun soil profile is a typical “Argic horizon”. Hence, by taking into account of other soil forming factors, the soil can be sorted as Hapli-Udic Argosol.