Abstract: In this study, adjustment factors of lactation milk yield to mature age and 305 day of milk yield were estimated for Holstein cattle reared in Polatli State Farm. A total of 3184 lactation records of Holstein cattle reared in Polatli State Farm between 1993 and 2006 were used. The factors for standardizing incomplete lactations to 305 day were estimated by taking into consideration nine calving age groups (2, 3, 4, 5, 6, 7, 8, 9, 10 and older years of age). The factors for standardizing to mature equivalent estimated by polynomial regression method are between the 20th month and 145th and older. For lactations less than 305 days, average of adjustment factors for 30, 60, 90, 120, 150, 180, 210, 240, 270, 300 and 305th days, were 8.70, 4.32, 2.82, 2.14, 1.69, 1.48, 1.31, 1.18, 1.09, 1.01 and 1.00, respectively. For lactations more than 305 days, the average of adjustment factors for 310, 340, 370, 400, 430, 460, 490, 520, 550, 580 and 600th days were 0.99, 0.93, 0.88, 0.84, 0.80, 0.77, 0.74, 0.72, 0.69, 0.67 and 0.65, respectively. Adjustment factors were estimated as 1.14 and 1.51 for the youngest cows (calving in 24th month) and for oldest cows (calving in 145th month), respectively.