These studies were conducted to determine the sex of preimplantation Hanwoo embryos produced in vitro using polymerase chain reaction(PCR). Y chromosome specific and bovine speicific DNA primers were synthesized and tested for embryo sexing. Bovine IVF embryos were produced in TCM 199 and CR1aa medium, and classified by developmental stages on Day 7 to 9. The effects of developmental rates to bovine IVF blastocysts on sex ratio were also investigated using PCR methods. The results obtained in this study were as follows; 1. Developmental rates to blastocyst from IVM/IVF embryos in TCM 199 and CR1aa medium for 9 days were 23.5 and 30.2%, respectively, and there was significant difference between the media(P<0.05). 2. Male to female ratio of early, mid, expanded and hatching balstocyst produced on Day 7 were 0.7:1, 1.4:1, 2.2:1, and 2.5:1, respectively, and male embryos was significantly higher proportion in expanding and hatching blastocysts(P<0.01). 3. On Day 8, male to female ratio of early, mid, expanded and hatching blastocysts were 0.6:1, 1:1, 2.5:1, and 2.7:1, respectively. Both expanded and hatching blastocysts obtained a significantly higher proportion of males(P<0.01). 4. The male : female ratio of early, mid, expanded and hatching blastocyst produced on Day 9 was 0.6:1, 0.8:1, 1:1, and 2.2:1, respectively. Hatching blastocysts had a significantly higher ratio of males(P<0.01). The developmental rate of IVM/IVF embryos to blastocyst for 9 day culture was higher in CR1aa than that in TCM 199 medium. For the sex ratio by developmental stages of IVF embryos, male ratio was higher in expanded blastocyst but female in early blastocysts.