This work was conducted to investigate a genetic relationship between part lactation period and total milk yield, in order to take advantage the information provided by a part record of lactation period (months) for evaluating of production efficiency which bulls can be evaluated for sire index. This study was included a monthly milk yield of 424 lactation records during 1978-2001 of pure breed Friesian in Mosul city (Iraq). The records were adjusted for fixed effects of calving season and monthly milk production records. The maximum average milk production was in the second month (395.6kg). The highest heritability (h²) estimated was (0.41) in the production of the third month, therefore, the production of the third month may be utilized for selecting animals for rapid improvement. While, the highest heritability estimated for cumulative monthly production was (0.32) for the first four month of lactation. Coefficients of genetic and phenotypic correlation between monthly milk production and total lactation production showed increasing trend with increase a cumulative months in milk production. The results of heritabilities and genetic correlation reveal that the part of milk production record was based on the third month of milk production was efficient as the total lactation production.