This study was performed to evaluate the effect of commercial swine mycoplasma vaccine(M+Pac(R),Schering-Plough Animal HealthCo.) at different vaccination time-points. Total 139 piglets were moved to experimental farm after weaning and were randomly allocated to 3 treatment groups and 1 control group. Piglets of 3 treatment groups(A, B and C) were vaccinated twice at 1 and 3 weeks(A group), 3 and 5 weeks (B), and 6 and 8 weeks(C), respectively. All vaccinates showed higher antibody titers compared with nonvaccinates, and maintained to finish(P<0.001). Performance and RBC/WBC count showed no significant difference between groups. Vaccinates at 6-8 weeks showed better effect on decrease of clinical sign compared with vaccinates at 1-3 weeks and 3-5 weeks and nonvaccinates. Also, lung lesion assessment showed significant difference between vaccinates at 3-5 weeks and 6-8 weeks, and nonvaccinates and vaccinates at 1-3 weeks(P<0.05). Vaccinates at 3-5 score in comparison with other groups. In conclusion, these results suggested that vacci-nation at 3-5 weeks or later may be more effective than earlier vaccination.