The variation of morphology and ultrastructure between fresh Siberian sturgeon sperm and frozen-thawed sperm were observed by transmission and scanning electron microscopy in this paper. The results showed that the sperm morphology and ultrastructure changed greatly after frozen-thawed. The length of acrosome, the ratio of middle head width to anterior head width and the width of midpiece increased significantly (P<0.05). The length of midpiece and posterolateral projections became short. there was significant difference between fresh and thawed sperm (P<0.05). About 30.5% of the sperms were damaged after frozen-thawed. The ultrastructure exhibition of freezing injure focused on: the posterolateral projections mixed with nuclear and the content of acrosome lost; the arrangement between acrosomal vesicle, subacrosome and nucleus became slack; the plasma membrane of sperm swelled, folded and disrupted; the membrance of endonuclear canals unclear; the membrane of mitochondria was cracked, dispersed and even mitochondria lost; the membrane of flagellum changed slack and released from flagellum. Some of the damage sperms lost the acrosome, midpiece and flagellum. The flagellum regularly disrupted from the posterior part of midpiece. Most damaged parts focused on the membrane-system of sperm, microtubular-system was still well. Freezing and thawing procedure were the key of cryopreservation. Study of Siberian sturgeon sperm ultrastructure would direct us choose the proper cryopreservant, extender and right method, including freezing and thawing procedure, to conserve the sperm.