Lactation records of dairy cows milked from January, 1969, to December, 1973, at Seo Sam Neung Ranch operated by National Agricultural Cooperative were standardized to 305-day, mature equivalent. Lactation yield was larger for cows calved in late fall and winter than those calved during summer. Cows calved in April, Way, and June were the lowest in frequency of calving. The phenotypic increases in milk yield for first lactation cows and third lactation cows were 27.4㎏ and 29.5㎏ per year, respectively. Older cows were more severely affected by summer calving than were first lactation heifers. It was observed that the correlation between first lactation yield and yields up to three lactations were highly significant. The negative correlation between the age at first calving and yields up to three lactations was in significant.