Abstract: Direct observation and the measurement of the temperature range of transparent microemulsion were used in studying the effects of different factors such as surfactant, co-solvent and water which are the basic elements of microemulsion on the physical stability of the microemulsion of diazinon. The study was based on the kind and concentration of every factors and the concentration of salt (NaCl).The results indicate that the optimum HLB of surfactant is 12~15 and the co-surfactant ethanol or methanol is effective to form qualified microemulsion. Different kinds of water are suitable to formulate transparent and steady microemulsion, but hard water to some extent has negative effect on the cold-storage stability of microemulsion. The temperature range of transparent microemulsion is -5~60℃ when the weight fraction of salt(NaCl) is less than 0.5%.