The process of sperm penetration into egg of Scylla serrata was examined with a scanning electron microscope(SEM)in this paper .The sperm contacts with vitelline membrane by its radial arms of the nucleus , and undergoes acrosome reaction immediately .During the process of acrosome reaction , the reversion of acrosonal vesicle , the forword-extending of the acrosomal tubule and the contraction of the radial arms were observed .The nuclear cup separates itself from the acrosome and is pull at the back of the acrosonal vesicle reversed .While the acrosome tubule passes through the egg membranes , the nucleus was carried into the egg . Because S .serrata has no micropyle , in the process of fertilization , a great number of sperms attach themselves to the egg and then several of them penetrate into the egg .The mechanism of the acrosome reaction and the action of the egg during the fertilization were discussed in this paper .