We analyzed the effects of age at first calving months of holstein cows and the number of delivery on the profit of the dairy farm by using the data provided by National Agricultural Cooperatives Federation. The data includes the milk production history of individual 68,231 cows, which were born in years from 2000 to 2001 and gave births until year 2009. We found that first delivery at 23~25 month old generates highest profit by leading to more ensuing deliveries, which is contradictory to the farmer`s expectation that first delivery by a more mature cow at older age would be more beneficial.