In order to study the circadian rhythm of juvenile Nile tilapia (Oreochromis niloticus), constant photoperiods were designed with 12 h in the light following 12 h in the darkness (12L:12D), 6 h in the light following 6 h in the darkness (6L:6D) and 2 h in the light following 2 h in the darkness (2L:2D), respectively. The results showed that:(1) when the photoperiod was 24 h, the Nile tilapia showed circadian rhythm, and its circadian rhythm was (24.3±0.2) h; (2) the circadian rhythms of the Nile tilapia still existed under sustained darkness and illumination, which were (25.1+1.1) h and (25.6+1) h respectively; (3) when the photoperiod was 12 h, the Nile tilapia still showed an obvious circadian rhythm that was (12.6±0.5) h; (4) when the photoperiod was 4 h, the swimming behavior of Nile tilapia still had an obvious circadian rhythm, the rhythm was (4.0±2.0) h. These results indicated that the Nile tilapia had endogenous circadian clock with 24 h cycle, however, compared with exogenous light regulation, endogenous circadian clock is relatively weak in tilapia. Exogenous light cycle is the main factor regulating the circadian rhythm of juvenile Nile tilapia.