Using Random Amplified Polymorphic DNA (RAPD ) technique with 20 kinds of random primers , genomic DNA of hybrid ( Oreochromis niloticus ♀×Oreochromis aureaus ♂) and its parents were analysed. The resultsshowedthatonlywiththetwokindsofprimers,OPZ -14 andOPZ -18 ,species2specific fragments were produced between parent fishes , even between different fish belonging to the same species , and some of them could be used as molecular markers to identify O. niloticus and O. aureaus. And 8 primers generate polymorphic products among hybrid fish and its parents. DNA fragments amplified from the hybrid fish were more abundant than those from its parents , which may be the result of more abundant genes in the hybrid fish , because of heredity or mutation. These results provide some molecular biological basis for species identification and heterosis effect in fish.