The objective of this study was to develop age adjustment factors, considering herd-year, parity, age at calving, month of calving and cow effects for production traits in dairy cattle, using 122,658 lactation records for milk and fat yield, and 51,947 lactation records for protein and SNF yield of dairy cows from 1985 to 1996. The results obtained were as follows; 1. Multiplicative age adjustment factors were developed for all production traits (milk, fat, protein and SNF yields), those were classified by calendar month of calving. 2. Productivity was increased according to age at calving, but it was decreased after mature age at calving. 3. The values of R2 for polynomial regression on BLUES of age at calving by month of calving were from 0.47 to 0.85 for milk, from 0.57 to 0.87 for fat, and being from 0.42 to 0.77, from 0.46 to 0.78 for protein and SNF, respectively, and the values in summer season were lower than those of other seasons. 4. Mature equivalent yields adjusted by the current factors were overestimated in the lst and 2nd parities. 5. Correlation coefficients between 305-day milk yield and mature equivalent milk yields adjusted by the current factors and new factors developed in this study were 0.92 and 0.98, respectively, and those for fat yield were 0.94 and 0.98, respectively. 6. Average deviations of mature equivalent milk yields adjusted by current factors from base mature milk yield by age at calving were ranged from -349.11㎏ to 1,335.47㎏ (average 342.95㎏), whereas those by new factors in this study were from -229.30㎏ to 284.16㎏ (average 51.73㎏). Therefore, overestimated yield was decreased considerably, and this result was the same for fat yield. 7. The trends of adjusted milk yields by current factors were similar to 305-day milk yields according to calendar month of calving, whereas those by new factors were approximately the same for each calendar month of calving. Therefore, differences in yield by month of calving seemed to be adjusted, and this result was same for fat, protein and SNF yield.