This study was carried out to investigate the effects of sperm concentrations and culture media on fertilization and development of in vitro matured pig oocytes. The concentrations of frozen-thawed sperm were 0.2×107, 2×107, 20×107 and 200×107/ml, respectively. Culture media were NCSU-23, HEPES-buffered (25 mM) NCSU-23, PZM-3 and PZM-4, respectively. Increasing the sperm concentration from 0.2×107 to 2×107/ml, significantly increased the penetration rate. Also, increasing the sperm concentration from 20×107 to 200×107/ml increased the penetration rate from 62.1% to 69.9%, respectively, with no differences between these two concentrations. A similar pattern was observed for polyspermic penetration and male pronucleus formation. The mean number of sperm per oocyte significantly increased in the 20×107/ml and again in the 200×107/ml sperm concentrations. The percentage of blastocysts from cleaved oocytes at the 2×107/ml sperm concentration was significantly higher than that at the 0.2×107, 20×107 and 200×107/ml sperm concentrations. The percentage of blastocysts from cleaved oocytes and the cell numbers per blastocyst were significantly higher in the HEPES-buffered NCSU-23 culture medium than in the NCSU-23, PZM-3 and PZM-4 culture media under a gas atmosphere of 5% CO2 in air.