After freeze-drying, the ultrastructure of boar sperms was observed by optical and electron microscopy. The in vitro development ability of the sperm was also examined with intracytoplasmic sperm injection (ICSI). The rate of male pronuclear formation was (68.52%), for cleavage (59.17%) and for blastocyst formation (19.16%) of the trehalose group (0.2 mol/L), significantly higher than those of the 50 mmol/L EDTA group (64.59%, 56.26% and 15.62%) and the control group (35.36%, 52.33% and 8.60%) (P<0.05). After storage for 60, 120 and 180 d at 4°C, no significant difference in the in vitro development was observed (P>0.05). The male pronuclear, cleavage and blastocyst formation after ICSI with freeze-dried spermatozoa incubated for 1 h was superior than those incubated for 2 h (P<0.05). No significant differences in the structures after stored at 4°C or ?20°C (P>0.05) were observed between the trehalose group and EDTA group. The percent of B grade freeze-dried boar spermatozoa in the trehalose group was higher than that of the EDTA group (P<0.05). Based on the ultrastructure observation, main cryogenic damage in freeze-dried boar spermatozoa was swelling, damage or rupture in the sperm acrosome, neck and tail.