This study was conducted to estimate the genetic and environmental effects on lactation yield and milk compositions in Holstein cows. The data analysis were the records of 159 cows rearing at Nation Animal Breeding Institute from 1990 to 1991. The least square means were estimated on milk and fat yield in lactation yield, and the percent of fat, protein, solids-not-fat and total solid in milk composition. The results obtained are summarized as follows: 1. The average yield of milk and fat in 305 days were $7,26.56{\pm}3,57.24$ and $254.65{\pm}44.94Kg$ and the percent of fat, protein, solids-not-fat and total solid were $3.69{\pm}0.43$, $3.32{\pm}0.41$, $9.15{\pm}0.49$ and $12.75{\pm}0.96$ and the coefficients of variation were 18.68, 17.64, 11.88 and 12.34% for milk yield, fat yield, fat percent and protein percent, respectively. 2. The effect of sires was highly significant at 1% level in milk and fat yield and fat percent, and significant at 5% level in protein and total solid percent. Among the sires, B, L and O sire were superior in milk yield with 7,571.22, 7,499.11 and 7,420.58 Kg, and A, F and K sire were superior in protein percent with 3.75, 3.64 and 3.65, respectively. 3. The effect of parity was highly significant at 1% level in milk yield, and significant at 5% level in fat yield. Among the parities, the 3rd parity was superior in milk and fat yield with 7,634.54 and 274.98 Kg, and the 4th and over was superior in fat and protein percent with 3.90 and 3.50, respectively. 4. The effect of calving seasons was highly significant at 1% level in milk yield, and significant at 5% level in fat yield, and the percent of fat, protein and total solid. Among the calving seasons, spring and winter were superior in milk yield with 7,310.31 and 7,364.57 Kg, also spring and winter were superior in protein percent with 3.68 and 3.52, respectively.