In China,more and more fishes were polluted by waterborne heavy metal.Four sizes of tilapia were cultured to different Cd exposure in order to study the influence of heavy metal concentration as well as the fish size on the Cd accumulation and transfer by the Orechromis niloticus.The median lethal Cd concentration (LC₅₀),Cd accumulation and transfer rate in fish and relative growth rate of the fish are analyzed.The results show that different age of tilapia had different response to Cd contamination.LC₅₀ in the juvenile fish was the lowest while that in 31.5±3.4 cm length fish was the highest.Exposed to low Cd²⁺ concentration (0.5 mg/kg),different size of fishes accumulated significantly different (P<0.05) quantity of Cd²⁺ in the same tissue.While exposed to high Cd²⁺ concentration,the Cd²⁺ accumulation in different size of fishes had no obvious difference.The Cd²⁺ transfer factor was significantly greater in low Cd²⁺ concentration than that in high Cd²⁺ concentration,for all fishes except fish with 27.4±2.9 cm length.High Cd²⁺ concentration inhibits the fish weight increase more than low Cd²⁺ concentration,in all age of fishes.In addition,different Cd²⁺ concentration had different effect on the same age of tilapia.In conclusion,both the fish size of tilapia and the Cd²⁺ concentration affected the Cd²⁺ accumulation and transfer in fish.