Aiming at exploring the sequence differences of IGFⅠ gene of Nile tilapia (Oreochromis niloticus NewGifts，ON), blackchin tilapia (Sarotherodon melanotheron，SM) and their First Generation of Hybrid (HB), investigating the relationships between the great differences of saltresistant property among the three genotypes, we cloned partial cDNA fragments of insulinlike growth factor Ⅰ(IGF-Ⅰ) from gill extracts through rapid amplification of 3′ cDNA ends（3′RACE）methods. The results demonstrated to be the IGF-Ⅰb gene cDNAs with length of 1 076 bp,1 075 bp and 1 079 bp respectively and comprising of a stop codon, a 546 bp open reading frame (ORF) encoding a 182 amino acid peptides(aa), and 3′untranslated regions（3′UTR）. The deduced amino acid sequence of IGF-Ⅰb precursor is composed of signal peptide, mature peptide（including conserved B, C, A and D domains consisting of 29aa,10aa,21aa and 8aa respectively）and E domain of 70aa. Results of the predicted secondary structures were typed as “mixed patterns”. The results of multiple alignments showed highly homologous with 75.8% to 100% amino acid identity. The B and A domains are highly conserved in all of the allied species. There are 2aadeletion between No.82 and 83 sites in C domain, 3aa and 1aadeletion next to NO.131 and No.159 site, respectively. Ala / Pro substitution occurred at No.133 of E domain of blackchin tilapia compared with ON and HB, which is identical to Oreochromis mossambicus and Oreochromis urolepis hornorum. We presumed that the differences of saltresistant property of ON, SM and HB may be due to the alterative splicing of the IGF-Ⅰ E domain.