Abstract: 17α-methyltestosterone (MT), a synthetic androgen drugs, has served as a specific medicine on fry culture and gender control in aquaculture. It found that there were MT residues in tilapia at the market, which would harm the human body as this androgen has great potential hazard on human health. Studies were conducted on the dissipation of MT in Nile tilapia Oreochromis niloticus (L.) fry that received dietary of MT to induce sex reversal under aquaculture model in last winter. Sexually undifferentiated Nile tilapia fry (8—10 mm, 9—13 mg) were fed on a diet containing unlabeled MT (200 mg/kg) for 30d to effect masculinization in the concrete tank, daily feeding was regular, water was changed once a week, shed was used natural to control the temperature at (28 ± 2)℃. After 30d, the fry were transferred to a pond with normal feed, MT in fry were dissipation in the natural environment. The fry were sampled in different time and frequency in the latter days, then MT residues in fry were measured by high performance liquid chromatography (HPLC), the retention time of MT was approximately 7.6min, the recoveries obtained from fry samples were above 83.2%, with the relative standard deviation were below 3.62%, the detection limits of MT was 20μg/kg. SPSS Statistics 17.0 combines EXCEL 2003 statistical software were applied to the data processing and analysis in order to get the dissipation rule of MT after tilapia sex reversal. The results showed that MT residues in tilapia fry almost maintained consistently during the sex reversal period, MT residues in tilapia fry body at roughly about (1937.17 ± 77.76) μg/kg, although overall trend of the dissipation was still downward while multi-peak phenomenon occurred in the dissipation of MT between two feedings. Besides, following the withdrawal, in the first 45d, MT had rapid reduction, in the subsequent 46-115d, the dissipation of MT was relatively slow, and ended approximately in 115d withdrawal, MT residues was not detected in tilapia fry from that day on, while the main weight of fry was (14.12±2.12) g. In general, the dissipation of MT was following the withdrawal slowly. By regression analysis of the MT dissipation curve, the exponential models were y = 1779.6e?0.0369x, R2 = 0.9332, where, Y = whole body concentration of MT in μg/kg fry body, and X = time in d after withdrawal of MT-treated feed (R2 = 0.9332). It suggested that the dissipation of MT fitted with exponential decline, and the dissipation was slowly in the latter. Though MT residues from tilapia sex reversal were not detected before marketing, but it could not be guaranteed as excessive use of MT and MT, after all, it was a synthetic hormone, its various impacts on organs of the fish itself would probably have indirect impact on human health. For safety’s sake, the use of MT in the aquaculture should be forbidden.