Abstract: A step-wise cooling schemes was employed to cryopreserve Crassostrea gigas sperm, and the sperm ultrastructure was observed by scanning electron microscopy and transmission electron microscopy. The results showed that there were no significant differences between frozen-thawed sperm and fresh sperm in the motility, fertilization rate and hatching rate. Both the fresh sperm and cryopreserved sperm had ultrastructural damages. The normal rates of the fresh and cryopreservated sperms were 84.5% and 73%, respectively. The cryopreserved sperm without damage had normal morphology in the plasma membrane, mitochondria and nuclear, the acrosome, and centriole, and the mitochondrion obtained integrity with well-developed cristae. The sperm cryodamages with damages had swelled or disrupted plasma and nuclear membrane, partially damaged nucleus and swelled, dislocated or disarticulated mitochondrion with degenerated or vanished cristae. The results showed that HBSS with at 1:4 dilution trehalose and 10% DMSO is the best condition for extender and cryoprotecant and for protecting the frozen-thawed C. gigas sperm, which will benefit the preservation of C. gigas and application of sperm cryopreservation skills.