Leptin, a product of the obese gene, is a multifunctional hormone secreted predominantly by adipocytes. It plays an important role in regulating energy balance and feeding behavior. Partial cDNA of leptin was isolated from genetically improved farmed tilapia (GIFT) (Oreochromis niloticus) through RT-PCR. The cDNA was 364 bp in length which encoded 96 amino acid residues. Homology analysis of the deduced amino acid sequence of fish leptin was carried out. Fish leptin showed a lower degree of homology. The deduced amino acid sequence of GIFT and Epinephelus coioides leptin was 80.6% identical, however the amino acids homology was below 40% when comparing GIFT with Rainbow Trout (Oncorhynchus mykiss), Medaka (Oryzias latipes), Zebrafish (Danio rerio) and so on. Leptin expression in different tissues was detected by real time PCR. The highest relative expression level was found in liver, which was 3 000 times as high as in muscle; high expression was also found in gonad and brain, which were 1 250 times and 450 times as high as in muscle, respectively. However, the relative expression level was very low in kidney, intestine and muscle.