This study aimed to assess the level of testicular damage by observing the changes in the diameter and epithelium thickness of seminiferous tubules in rats that exposure to nicotine per inhalation. Thirty adult male rats were used and divided into five equal groups and treatment as follows for 20 days; Control group NaCl 0.9%, P1 nicotine 0.5 mg/kg, P2 nicotine 1.0 mg/kg, P3 nicotine 2.0 mg/kg and P4 nicotine 4.0 mg/kg. All groups were given treatment per inhalation for twenty days. At the end of treatment and the rats were sacrificed testes were collected for histopathological preparation. The testes were processed for routine paraffin embedding and staining and the sections were examined for histopathological changes. There results showed that nicotine administration induced varying degrees of structural damage to the seminiferous tubules, as the decreased in diamater and epithelium thickness of seminiferous tubules. The diameter and epithelium thickness of seminiferous tubules in four experimental groups reduced compared to the control group. This study proves that nicotine administration does decreases the spermatogenesis of rats by reducing the diameter and epithelium thickness of seminiferous tubules in testes. It also proves that the level of testicular damage is directly proportional to the dosage of nicotine administrated to male rats.