BACKGROUND: Phenol is a toxic organic chemical found in many foods and chemicals in our environment. OBJECTIVES: Regarding to the wide use of phenol and its harmful effects, this study was done to determine the effect of pure phenol on morphometrical and histometrical structure of testis. METHODS: 24 mature male Syrian mice divided to one control and 3 treatment groups that received pure phenol at 30, 75 and 100 mg/kg doses through gavage during 35 days. Finally, body, testis and tunica albuginea layer weight, gonadosomatic index (GSI) and length, width and thickness of the testis were measured. For histometrical assessment, the diameter of seminiferous tubules and the thickness of germinal layer were measured. RESULTS: Body weight in 75 and 100 mg/kg doses (2.02±4.09 and 2.33±3.35g, respectively) had significant decrease (p<0.05) comparing with control group (10.16±3.97g). The difference in weight, length, width and thickness of testis between treated groups and control group was not significant, but the GSI in 30, 75 and 100 mg/kg doses (0.53±0.11, 0.53±0.07, 0.5±0.07%, respectively) had significant increase (p<0.05) comparing with control group (0.088±0.0083%). Weight of tunica albuginea layer had increased significantly (p<0.05) only at dose of 75 mg/kg (0.01±0.006g) comparing with control group (0.002±0.002g). There were significant differences (p<0.05) in diameter of seminiferous tubules in control group (100.42±12.41µm) comparing with 30 mg/kg (130.18±49.14 µm) and 75 mg/kg doses (156.89±53.36µm) and thickness of germinal layer in control group (47.96±18.42 µm) comparing with 30 mg/kg (37.18±14.44 µm) and 75 mg/kg (30.13±50.04 µm) doses. CONCLUSIONS: Pure phenol can cause changes in body weight, GSI and histometrical structure of testis.