Genetic analysis of lactation traits in Friesian cattle was carried out on data of 4849 records of 1544 cows sired by 254 bulls. Yields of milk of 305-day (305 MY) and total lactation (TMY) and length of lactation (LP) and dry period (DP) were the traits examined. Two methods of gross comparison (GC) and polynomial regression analysis (PRA) were used to calculate different sets of multiplicative factors to standardize milk yield of Friesian cattle to mature equivalent basis and to minimize environmental variation due to days open. Means of 305 MY, TMY, LP and DP were 2384 Kg, 2577 Kg, 322 days and 83 days, respectively. Effects of year and season of calving, age at calving and days open were generally significant source of variation for milk traits studied. A curvilinear relationships for age of cow could b4=, fitted on the data of milk yield of Friesian cattle raised in Egypt. Al]. lactation traits increased in a curvilinear manner (P