This paper deals with the cross fertilization between the female redcarp (Cyprinus carpio L.) and the male grase carp (Ctenapharyngoden idellus Cuv etVal). Even though the red carp and grass carp belong to differnet subfamilies, thefertilization agrees with the general law of cyprinidae fish. A single sperm enters theegg through the micropyle. The approperiate time of fertilization is at the metaphaseof the second meionis. Seven minutes after fertilization, the sperm-aster appears,thenthe male and female pronuclei form respectively and draw close gradually. Fourtyminutes after fertilization, the male and female pronuclei fuse to form the nuclearzygote (at the water temperature of 21℃). By the 45--50 minutes. the first cleavageoccurs. This indicates that the sperm of grass carp can not only activate the matured eggof the red carp but also attends the composition of herdity matter, which is biologicalbasis for the vitality of the hybrid. In the experiment, it is found that polyploid,monoploid and gynogensis may occur at the same time among the same group of thefertilized eggs under some specific conditions. Finally, a comparative study on thefertilizing process of some common carps as well as their hybridization is also made.