Abstract: A complete diallel mating system with Oreochromis niloticus，O.aureus，and their hybrids，and additive dominant genetic model were used to analyse heterosis in different fish density environments of concrete pond and small cage. The results indicated that (1) Heterosis of growth related traits in F1 hybrid was higher than in F2, and varied in different trait from 15.1% to 30.6% in F1, but decreased in F2 with the range of 7.6% to 15.3%. Mean heterosis of these growth related traits were all positive in F1, but negative in F2. (2)The genotype effects and heterosis of all these traits had interaction with environment. In cages with high density of fish, the heterosis of these traits were much higher, and in concrete ponds with low fish density they were much lower, suggesting that environment effects should be considered in utilizing heterosis of tilapia.