Abstract: Objective　To investigate the effect of continuous light on the puberty initiation in female rats. Methods　 Forty 21-day-old female rats were weaned and randomly divided into two groups, continuous light group and control group. Continuous light group were exposed to light 24 hours a day while control group were exposed to 12h:12h light:dark cycle. After one week, E2, LH, FSH, PRL, T and MT were measured. MT was measured at different time points including zeitgber time (ZT) 2, ZT6, ZT10, ZT14, ZT18 and ZT22. The age at vaginal opening and first diestrus were observed. Behavior evaluation was carried out when rats were 40 days old. Rats were sacrificed at the age of 45 day and pineal gland was observed morphologically. Results 　The levels of E2, LH, FSH, PRL and T were significantly higher in continuous light group than in control group (P<0.05). MT level in control group during dark time was significantly higher than that in continuous light group (P<0.05). No significant difference was observed in MT level during daytime (P>0.05). The age at vaginal opening and first diestrus were earlier in continuous light group than control group (P<0.05). Open field test showed that the rats in continuous light group were more anxious than in control group. HE staining of pineal gland showed decreased and loosely distributed pineal cells in continuous light group. TEM test found that the nuclei in continuous light group had apoptosis-like changes and other ultrastructural changes. Conclusions　Excessive light can inhibit the secretion of melatonin and enhance the activity of sexual gland, which can result in the earlier sexual development.