This study was carried out to estimate the effects of breed and environment such as sex, test station, test year, test season, parity, initial and final weight on average daily gain, age at 90kg, backfat thickness, feed efficiency, lean percent and selection index on the basis of the performance data collected from 25,790 pigs of Duroc, Yorkshire and Landrace breeds which were performance-tested at the Korea Swine Testing Station from 1991 to 2002. The results obtained in the study are summarized as follows; 1. The means of the major economic traits were estimated as 959.95${pm}$0.699g for average daily gain, 138.36${pm}$0.072days for age at 90kg, 1.41${pm}$0.001cm for backfat thickness, 2.33${pm}$0.001 for feed efficiency, 56.71${pm}$0.018% for lean percent and 221.65${pm}$0.113 for selection index. 2. The effect of breed was statistically significant for all studied traits. Briefly, Duroc showed the best performance for the average daily gain and age at 90kg. Landrace had the best performances for the backfat thickness and lean meat percent. In feed efficiency and selection index, Yorkshire had a better score than other breeds. 3. The least-squares means of female and male for the traits studied were 923.05${pm}$1.289g and 974.53${pm}$0.856g for average daily gain, 139.74${pm}$0.145days and 137.21${pm}$0.097days for age at 90kg, 1.49${pm}$0.002cm and 1.39${pm}$0.002cm for backfat thickness, 2.43${pm}$0.002 and 2.28${pm}$0.002 for feed efficiency, 56.43${pm}$0.034% and 56.81${pm}$0.023% for lean percent and 211.37${pm}$0.194 and 224.61${pm}$0.129 for selection index. Therefore, males were superior to females for all traits examined. 4. The effect of test station was statistically significant for all traits except for selection index. Performances for age at 90kg, backfat thickness, feed efficiency and lean meat percent collected from Test station 2 were higher than those from Test station 1. However, Test station 1 showed better average daily gain. 5. The initial weight and final weight included as a covariate in this study had a significant influence on average daily gain, age at 90kg, backfat thickness, feed efficiency and selection index. From the absolute values of the estimated regression coefficients, it was inferred that the final weight had greater effect for the investigated traits than the initial weight.