Ratio factors for extending partial records to 305day were estimated on 3,740 complete lactation rewrds collected from 6 Holstein herds from 1969 through 1984. The results obtained are as follows: 1. The single monthly ratio factors were higher in the late stage than in the early stage, and higher for the 1st parity than the 2nd, 3rd and later partities. The value of R² of the factors was relatively high in the middle stage (4th-7th month). 2. The cumulative monthly ratio factors were also higher for the 1st parity than the other parities, and lower in July through October than in the other months. The value of R2 of the factors increased as monthly tests continued; and over 0.80 if the number of monthly test record included more than 5. 3. The ratio factors for the period of unknown part on the last monthly yield decreased while the value of R² of the factors increased as the period of unknown part became short.