The objective of this study was to investigate the relationship between somatic cell score (SCS) and milk yield in different stages of lactation for cows in different parities. Records between June 2003 and January 2014 from 209,781 cows in lactations one to nine in 845 herds, comprising 2,500,407 monthly test-day (TD) records extracted from the animal breeding center, were used. The MIXED procedure of SAS software was used to investigate the effect of SCS on milk yield in different stages of lactation. Defined model considered herd, year-season of calving, month of TD, weeks in lactation and previous dry period length as fixed effects and calving age and SCS as covariate. Lactations were divided into six stages and analyses were performed within each stage. Also, different lactations were analyzed separately. The amount of daily milk yield loss associated with increased SCS was higher with increased number of parity and also later in lactation (especially after peak in week nine). The regression coefficient for milk yield on SCS was -0.539 to -0.635 in different stages of lactation in first parity cows, while that was between -0.777 to -1.053 in third lactation cows.