To evaluate the salt tolerance of four strains of tilapia Oreochromis mossambicus, O. hornorum, cross hybrid (O. mossambicus♀×O. hornorum♂) and reciprocal cross hybrid (O. hornorum♀×O. mossambicus♂) at molecular level, expression profile of heat shock protein 70 gene (HSP70) in four tissues (gill, kidney, pituitary and muscle) of the four strains of tilapia under salt stress was studied using RT-PCR method. The results showed that at salinity 22, HSP70 gene expression in the four tissues of those tilapias increased first, peaked at 24 h and then dropped to initial level at 48 h, and there was no significant change in its expression level when the salinity was increased to 35. There were differences in the expression level of HSP70 gene among different strains of tilapia and among the four tissues under the treatment of salt stress. The expression level of HSP70 gene was not up-regulated in the gill of cross hybrid throughout the treatment. The expression level of HSP70 gene peaked earlier in the kidneys of cross and reciprocal cross hybrids compared to that in the parents at salinity 22. The expression level of HSP70 gene increased slowly from 0h to 48h, and peaked at 48h in the pituitary of reciprocal cross hybrid at salinity 22. The expression level of HSP70 gene in the muscle of O. mossambicus increased rapidly at salinity 22 and maintained high at salinity 35. In conclusion, the expression level of HSP70 gene was closely related to salt tolerance and stress level of tilapia. The expression level of HSP70 gene in the kidney of tilapia under salt stress was in positive correlation with their salt tolerance and it can be used as a potential molecular indicator for evaluating salt tolerance of tilapia.