The present study aimed to investigate the effect of thiamethoxam on the physiological and histological status of male mice reproductive system. This study was carried out at College of Veterinary Medicine-Baghdad, from August to December 2016. Fifty adult males Swiss albino mice, aged two months, were divided into five equal groups. First group immunized with B. abortus antigen. Second group was dosed orally thiamethoxam. Third group were subjected to same treatments in the 1st and 2nd group. The 4th group injected virulent B. abortus. The 5th group acted as negative controls. All animals were euthanized and blood samples were taken for hormonal analysis and pieces of testes, epididymis were fixed in 10% normal buffer formalin for routine histopathological examination. Results showed that thiamethoxam significantly decreased the sperm viability, motility and testosterone level. In conclusion, thiamethoxam possess toxic effect for spermatogenesis in mice. To avoid its harm on human, the use of thiamethoxam should be expertised.