The present study was conducted to investigate the optimum conditions of in vitro fertilization by ejaculated boar spermatiozoa capacitated in a defined medium. When fresh ejaculated boar spermatozoa were preincubated for 4h with elevated sperm concentration fertilization rates were markedly increased. The fertilization rates by spermatozoa preincubated at the concentrations of 2, 4, 10, 20 and 40×10^8 cells/ml were 12, 30, 50, 76 and 82%, respectively, However, when spermatozoa preserved for 10-12h at 20℃ were preincubated for 4h with elevated sperm concentration fertilization rates were markedly decreased. The fertilization rates by spermatozoa preincubated at the concentrations of 2, 4, 10, 20 and 40×10^8 cells/ml at preincubation were 91, 77, 68, 44 and 8%, respectively. When oocytes matured in vitro were inseminated with spermatozoa at the concentration 1 × I0^6 cells/ml, rates of fertilization and polyspermy were 97 and 87%, respectively, after culture for 20h without changing medium. When oocytes were inseminated with spermatozoa at the different concentrations of 1, 0.5, 0.25 and 0.1×10^6 cells /ml and the medium was changed 5h after insemination, the fertilization rates were 75, 73, 61 and 30%, respectively. The rates of polyspermic fertilization were 58, 47, 40 and 47%, respectively, when oocytes were inseminated with different concentrations of spermatozoa, and there was no difference in the rates of polyspermy fertilization among groups with changing medium. The present results suggested that both sperm preincubation with high concentration and sperm preservation at 20℃ for 10-12h were effective for capacitation ejaculated boar spermatozoa and that a complete block of polyspermy was not achieved in this in vitro fertilization conditions.