This study was performed to investigate the effect of vent on the marginal discrepancy of PFM base metal castings. Eighteen identical wax patterns of a maxillary 3 unit anterior bridge were fabricated. Nine cases out of them were vented with $phi$2.5mm sprue wax, for direct comparison as to the marginal discrepancy of the castings with the other non vented 9 ones. The mean value of marginal discrepancies of the vented castings was $85.02{pm}25.57{mu}m$, and that of the non vented was $163.21{pm}69.88{mu}m$ respectively. Statistical analysis revealed that marginal discrepancy of the vented casting was significantly reduced(p<0.05).