With UF-resin as wall material, chlorpyrifos microcapsules was prepared by in-situ polymerization. The effect factors on the particle size and encapsulation efficiency of microcapsules were investigated and optimized. The result showed when the mass concentration of the emulsifier Tween-20 was 6%, the size of chlorpyrifos microcapsules were suitable. The amount of UF-resin has slight effect on the encapsulation rate. However, the effects of acidification and curing time on encapsulation ratio were quite notable，and the optimal acidification and curing time were 75 min and 90 min respectively. Chlorpyrifos microcapsules prepared under the optimum condition were spherical and smooth with the particle size of 10 μm, entrapment efficiency of chlorpyrifos microcapsules were above 95%.