Random Amplified Polymorphic DNA (RAPD) was used to detect the variation of four successively selected generations( F_6,F_7,F_8,F_9 ) of GIFT strain of Nile tilapia(Oreochromis niloticus).The results showed that: (1)172 loci (300-2800 bp ) were examined by 17 random primers from 88 individuals of four generations. (2)The proportions of polymorphic loci of F_6,F_7,F_8,F_9 were 23.4%,21.1%,19.4%and 18.3% respectively,decreased generation by generation,but no significant differences among generations (U test, P>0.05);the Nei genetic identities of F_6,F_7,F_8,F_9 were 0.896 1,0.908 4,0.918 6,0.925 4 respectively ,increased generation by generation,but no significant differences among generations(t- test, P>0.05);Shannon genetic diversity indices of F_6,F_7,F_8,F_9 were 0.148 5,0.131 4,0.117 8,0.108 3, decreased generation by generation, but no significant differences among generations(t -test, P>0.05).(3)The contribution of the variation among four generations to the total variations of four generation was 23.1% in Shannon genetic diversity index and 12.5% in AMOVA analysis. The above results showed that, although a selection of four generations did not cause a significant difference of genetic diversities, it caused a clear trend in genetical purification, and the F_8 and F_9 generations have reached a rather high genetic stability as a genetic improved variety.