The changes of grain size distribution and microstructure of loess after rainfall were investigated inthis study,as well as their effects on the initiation of mudflow.A simulated rainfall experiment was conducted in field for this purpose.We found that after rainfall,the volumetric percentages of fine and rough clayparticles increase approximately 22% and 18%,and those of silt and fine sand decreased 7.6%and 3.3%,respectively,in the samples collected from the sediment deposits.Consequently,the grain size of residual loess on the slope coarsened and the connection of loess skeleton became loose after rainfall.Such changes ofgrain size and microstructure of loess induced by rainfall implies that both of the shear and pressure resistanceof loess will decrease after rainfall.Thus,the probability of the mudflow initiation increases.Once the balance between the gravitational component of loess along the slope and the shear resistance is broken as therainfall penetrates,the loess begins to slip and possibly develops into mudflow.Preventing the loess on theslope effectively from being coarsened is the key to mitigate water loss and soil erosion.