Abstract: Soil particle size distribution is one of the key physical factors regulating soil fertility.To quantify the relationship between the depth of humus horizon and particle size composition,this study made a field survey of the depth of humus horizon and measured soil particle size in tillage horizon on a typical slope farmland in the black soil region of Northeastern China.Depth of humus horizon increases from up slope to down slope,but this trend is often interrupted by local topographic changes.There are no consistent trends in particle size composition from upper slope to lower slope,except that the sand content is higher at top slope than in other slope positions.Soil particle size composition of tillage horizon is significantly related to the depth of humus horizon.Both clay and silt contents are exponentially increased with the depth of humus horizon,and the sand content is exponentially decreased with the depth of humus horizon.However,when the depth of humus horizon was thicker than 60 cm,the particle size composition did not change significantly with further increasing depth of humus horizon.