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First lactation test day records on 36487 cows, collected during years 1991-2002 in the Iranian Animal Breeding Center, were used in this research to investigate the trend of milk yield over lactation time, and to study the effect of factors determining the shape of lactation curve. Seven mathematical functions were fitted to individual records and based on the coefficient of determination, the incomplete gamma function was found to be the best to describe the lactation curves and therefore was selected as the most suitable function. Lactation curve traits that were employed in the analysis included a scaling factor associated with yield at the beginning, the inclining and declining slopes before and after peak yield, peak yield, DIM to reach peak yield, persistency and total milk yield, estimated from parameters of incomplete gamma function. Herd-year-season, DIM at the first and the final records, and days open had significant effects on the mentioned traits (P<0.01). The phenotypic correlation between total milk yield and all traits, except for the inclining slope, were positive. Total milk yield was highly correlated with peak yield (r=0.90).