The purpose of this study was to investigate the effect of sex, breed, year of birth, month of birth, parity of sows and litter size on average daily gain, age at 90kg and backfat thickness in swine. The data used in this study were obtained from 7,650 heads of Duroc, Hampshire. Landrace and Yorkshire boars and gilts tested on an age-constant basis at a swine breeding farm in Chungchongbuk-Do from March, 1984 to February, 1987. The results obtained in this study are summarized as follows: l. The least-squares overall means of the traits studied were. 0.780±0.003kg for average daily gain, 155.6±0.3 days for age at 90kg and 1.679±0.008cm for backfat thickness. respectively. 2. Males had significantly higher average daily gain and earlier age at 90kg and thinner backfat than females. 3. Among the breeds compared, the Duroc had the highest average daily gain and reached 90kg at the earliest age. The Hampshire had the thinnest backfat. 4. Year of birth and month of birth affected the three traits studied significantly. The pigs born in June and July had comparatively higher average daily gain and reached 90kg at an earlier age than those born in other months. The pigs born in March and April had lower average daily gain, and the pigs born in December and January had the thinnest backfat. 5. The effect of litter size was statistically significant for none of the three traits studied. 6. The pigs in 2nd parity had the highest average daily gain and reached 90kg at the earliest age. Backfat was thinnest in the 3rd parity.