The fatty acid synthase(FAS)was cloned from liver of gift tilapia(Oreochromis niloticus).The obtained partial cDNA was 557 bp,which encoded 185 amino acids.Compared with other species FAS,the gift tilapia FAS shared 62%-82% identities.To study the FAS activity and gene expression affected by different dietary lipid levels,three groups of different dietary lipid levels(3.71% group,7.67% group and 16.55% group) were set up with the same nitrogen content.After feeding for 90 days,the fish fasted for 48 hours and then the liver FAS activity and FAS mRNA expression level in liver and muscle were studied;Further,the FAS mRNA expression level in liver was examined after refeeding with different lipid level dietary,at 6,12,24,48 hours.Results showed that FAS activity in liver had no significant change with different dietary lipid levels(P>0.05).The expression level of FAS mRNA in liver was significantly higher than that in muscle(P<0.05).However,the expression level of FAS mRNA in both tissues of liver and muscle was significantly declined with the raising of the dietary lipid level(P<0.05).After refeeding for 6 to 48 hours,the expression level of liver FAS mRNA was significantly declined in each group(P<0.05).Those results indicated that the expression level of liver FAS mRNA was significantly higher than that in muscle.The dietary lipid level could inhibit the expression level of FAS mRNA,and the tendency is:the higher lipid level,the more obvious inhibition.After refeeding for 6 to 48 hours,the expression of FAS gene was impeded.