Age-correction factors were developed on 4,538 complete lactation records from registered Holstein cows in the Animal Improvement Association of Korea from 1969 to 1983. These factors were computed by use of the gross comparision method for separate two seasons and overall season because yields of lactations initiated in season 1 (December through June) were about 6% larger than amounts produced by cows calving in season 2 (July through November). The new factors were smoothed across ages to eliminate minor fluctuation in yield that result from small number of records available in some age-month groups. It was concluded that the younger cows would lead to have 1 to 3 percentage of biased production records and that the older cows would have 5 to 7 percentage of biased production records if USDA age adjustment factors were used to the Korean field data.