BACKGROUNDS:There are some evidences that cholesterol can affect fish reproductive system. OBJECTIVES: In this study the effect of high diet cholesterol on fish sex hormones and gonads weight was investigated. METHODS: A randomized experiment was conducted on 90 immature common carp. The fishes were divided into three groups with three replicates each (control, 0.5% cholesterol diet, and 1% cholesterol diet). After one-month feeding, progesterone, estrogen and testosterone were measured. RESULTS: The level of progesterone in control group, 0.5% cholesterol and 1% cholesterol diet were 1.83±0.42, 2.45±0.38 and 2.62±0.52 ng/ml, respectively. Also the level of estrogen in control group, 0.5 % and 1% cholesterol diet were 3670.34±186.26, 3791.20±98.48, and 3836.78±81.74 pg/ml, respectively., while those of testosterone levels were 1.75±0.319, 2.09±0.425, and 2.25±0.321 ng/ml, respectively. The highest and the lowest body weight and gonad weight were observed in fish fed %1 cholesterol and the control, respectively. The results showed positive effects of cholesterol on sexual hormones, the 0.5 % cholesterol diet and the 1% cholesterol diet did not show any significant difference in the sexual hormones levels. CONCLUSIONS: The results indicated that the effect of cholesterol on the these sex hormones, gonad weight, body length and body weight was significant (p<0.01). We can conclude that diets of 0.5% cholesterol can be used to increase the amount of sexual hormones in common carp.