Abstract: To study the expression mechanism of insulin- like growth factor 1(IGF1) gene and lay the theoretical basis for molecular selection of tilapia breeds，IGF1 gene mRNA level was analyzed during different developmental stages of Nile tilapia (Oreochromis niloticus). The expression level of IGF1 gene in liver and muscle were quantified by Real-time PCR on day 0, 20, 40, 60, 80, 125, 145, 165, 185, 205. The results showed that development of body weight in tested fish were divided into different stages: rapid growth stage (RGS) and stationary growth stage (SGS). In RGS, the hepatic gene expression level of IGF1 was higher than that in SGS, sharing the same trend with the tendency of body weight development. In muscle, the IGF1 mRNA level was evidently lower than in liver. In conclusion, IGF1 mRNA level differed with temporal and tissuespecific characteristics. The developmental changes of hepatic IGF1 mRNA might be a key factor to different growth speed in development stages in Nile tilapia. Compared with hepatic pattern, muscular IGF1 gene expression was different, which implied a distinct functional pattern of IGF1 in muscle.