Mechanical properties of soil including resistance to pressure, shearing, and deformation, internal pressure in relation to soil moisture content were studied with model experiment of paddy soils in rice stubble field and in wheat stubble field under laboratory condition. The mechanical strength of paddy soil in rice stubble field changed greatly with changing soil moisture content.This is due to a rapid development of atcraction of molecular resulting from intensive shrinkage. Paddy soils in wheat stubble field,however,were found to show less change in this respect. When soil moisture content was high, the internal friction of clay soil approached zero.Its shearing strength was effected by soil cohesion only and its internal friction increased with the change of texture from heavy to light. When soil moisture content dropped, the internal friction of all the soils studied increased rapidly and was effected by the strength of aggregates and the degree of roughness of the shearing surfaces. When soil moisture dropped to the lower part of plasticity, fragility of soil began to occur, but a certain degree of plasticity and viscosity still remained. Fragility may occur at high content of soil moisture to soils with light texture and weak arranxement. As the change of a soil from plastic to solid is a continuous process, a larger range of soil moisture content should be allowed for the demarcation between a plastic and a solid soil. Since the internal pressure of the paddy soil in rice stubble field increased rapidly upon decrease of soil moisture content, especially so in the case of clay paddy soil. Hence, for cultivation on such soils a higher optimum soil moisture content is suggested for the main purpose of tillage and to mmmize soil plasticity and viscosity. On account of this, it is necessary to have the plow to form soil blocks of smaller primary shear planes, to have a higher speed of plowine. to. have the land olowed twice. first far is advance of olantine and then immediately before planting, and to have a larger contact surface between soil and tire so as to increase the draft force of the tractor.