Northern pike (Esox lucius) is a very popular edible fish with high quality and an economically important fish. In China, northern pik is only distributed in the Ertixche River in northern Xinjiang. Northern pike females grow faster than males. To investigate and develop the feminization induction method on northern pike, 750 fries of this fish were divided into five groups in this study. Among them, four groups were used as experimental groups, and fed with Tubifex hattai, which had been soaked with 20 mg/L 17β-estradiol, starting from different developmental time (10 d, 20 d, 30 d and 40 d, respectively after hatching). The other one group was used as control group, and fed with normal T. hattai. All fish in experimental groups were fed with 17β-estradiol for 30 d, and then all the fish in the experimental groups and the control group were fed normally. On 206 d, survival situations of the fish in all groups were counted, and then all the fish were dissected, gonads were removed to perform histological observation. The results showed that feeding with T. hattai, which had been soaked with 20 mg/L 17β-estradiol, had very obvious lethal effect on northern pike fries, and made the survival rate decreased from 43.5% in the control group to 14.3%±8.3%. Higher ratios of female to male were obtained in 30 d treated group and 40 d treated group, especially the highest ratio, which reached 15:7, was obtained in the 30 d treated group, so it can be concluded that the key time point of gonad differentiation of northern pike may be in the period of 30-40 d after hatching. There were high proportions of hermaphroditism in both experimental and control groups, that is, a large number of primary oocytes (phase Ⅱ) and spermatocytes appeared simultaneously in the same gland, and moreover, these oocytes showed marked degeneration. It is presumed that there was a protogynous juvenile hermaphroditism in the process of sexual differentiation of northern pike. This study will provide certain basis for production of all female northern pike fries using hormone induction techniques and further research on sex determining mechanism of northern pike.