Abstract: A study was undertaken to evaluate the effect of the densities on the energy conversion efficiency of Litopenaeus vannamei in June~July, 2004. The experimental juvenile shrimps of 5.0 to 6.0 cm per individual and 2.75 to 3.20 g per individual were reared in 9 tanks (0.10 m3) for three weeks with three different densities, 50, 150 and 400 indm-3, respectively. Each treatment was triplicate. The feed ratio was 4% to 7%. The results showed that the energy conversion efficiency were 19.31%1.01%, 22.56%2.69% and 15.31%2.49% in 50 indm-3 group, 150 indm-3 group and in 400 indm-3 group, respectively. The highest energy conversion efficiency was observed in shrimp reared in 150 indm-3 group. It indicated that the energy conversion efficiency was significantly affected by density.