Background and purpose: Diazinon (DZN) is an organ phosphoric pesticide that can cause damage to the cells and tissues while producing oxidative stress. Therefore, this study was conducted to determine the effect of diazinon on testosterone and testicular tissues in mice. Materials and Methods: In this in-vitro study, 28 adult male mice with a mean age of 12 weeks were randomly divided into four groups: control, sham and experimental groups 1 and 2. Experimental groups 1 and 2 received diazinon toxin 0.5 and 1.5 mg / kg body weight for gavage for four weeks, respectively. The control group did not receive any substance, and the sham group received the solvent of the poison. The rats were killed seven days later and the testes were examined histologically. Concentration of testosterone was measured by radioimmunoassay method. Results: The number of germinal cells and tubes diameter of Seminifer in the experimental group 2 was significantly lower than the control group (P <0.05). Conclusion: The results indicate that testicular tissue is susceptible to diazinon and its effect is dependent on dose.