Aim of the present study was to determine the effect of the lactation number on milk production and to detect the most economical lactation period in Holstein dairy cows. Materials and The animal materials of the study were 392 Holstein dairy cows with a similar dry period and lactation length. The cows were divided into 6 groups according to the number of lactations. The groups were formed as 1st lactation (Group 1; n=67), 2nd lactation (Group 2; n=124), 3rd lactation (Group 3; n=51), 4th lactation (Group 4; n=62), 5th lactation (Group 5; n=41) and 6th lactation (Group 6; n=47).Average lactation milk yields for the 1st, 2nd, 3rd, 4th, 5th, and 6th lactations of the cows used in the study were 7860.03 kg, 9010.02 kg, 10207.15 kg, 10165.89 kg, 8430.57 kg and 8069.78 kg, respectively. It was determined that the highest milk production and highest daily income were obtained during the 3rd and 4th lactation periods (p<0.05).In conclusion, milk production increased in the first three lactations, 3 and 4 lactation remained stable and decreased in 5th and 6th lactations. But, regardless of the number of lactations during the first 6 lactation periods, it is considered economical to use healthy cows for milk production in dairy farms.