Abstract: The effect of hydrocarbon chain of co-surfactant,chlorpyrifos solution (oil phase) content,chlorpyrifos content in oil,temperature,and water hardness were investigated by surfactant/cosurfactent/oil/water pseudo-ternary phase diagram to evaluate the factor effect in forming chlorpyrifos microemulsion.It was found that TX-10 could form microemulsion or liquid crystal phase without the assistant of co-surfactant.The emulsifying effect can be enforced by mixing TX-10 with anion surfactant 500#,with optimum ratio of 1∶ 1(m/m).Pseudo-ternary phase diagram of the mixing TX-10/500# surfactant system revealed that the area of microemulsion was decreased distinctly with the increasing of oil phase,but affected slightly by the content of raw chlorpyrifos in oil phase,experiment temperature and water hardness.