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.