Cdc2(Cyclin Dependent Kinase, namely CDK1) encoded by cdc2 gene and CyclinB combination regulates G2/M transition. To find out molecular mechanism that diploid hybrid fish could produce diploid gamete, the full length cDNAs of cdc2 in the third gynogenetic generation (G3), red crucian carp( Carassius auratus red var.), triploid crucian carp and allotetraploid were obtained by PCR and rapid amplification of cDNA ends. Our data showed that all the cDNAs of cdc2 gene in the four different ploidy fishes encode a protein of 302 amino acids containing a domain (PSTAVRE) which combinate Cyclins. A high homology of 97.6% of the Cdc2 protein can be drawn by comparing the amino acid sequences in this four fishes, which indicating the higher conservative function and evolution of Cdc2 protein in this four fishes. A comparative expression pattern of cdc2 in early-stage gonads of G3 and different ploidy fishes was carried out by Real-time PCR using specific primers against the same sequences of coding regions in the four fishes. The results showed that the expression of cdc2 in the ovary of G3 was higher than those of red crucian carp and triploid crucian carp, while lower than that of allotetraploid, which, at the molecular level, indicates existence of polyploid oogonia in early-stage gonads of G3. The higher expression of cdc2 in G3 suggests that consecutive S-phase replication may occur without intervening mitosis, which might be related to the formation mechanisms for the diploid eggs generated by diploid hybrids.