Reproduction and milk production are the principal factors that are influencing dairy farm profitability. The dairy industry in Iran has changed dramatically in the last decade. The shift toward more productive cows and larger herds in Iran is associated with a decrease reproductive efficiency (Heravi Moussavi et al., 2004). Increased knowledge about the principal causes of reduced fertility is essential. The root cause of the declining fertility is probably a combination of a variety of physiological and management factors that have an additive effect on reproductive efficiency. Dairy cattle are inseminated and pregnancy is established while dairy cows are lactating. Based on the analyses of large datasets, there is clearly an antagonistic relationship between milk production and reproduction in dairy cattle (Lucy, 2001). It was shown that the hazard ratio for cumulative first 60-day milk yield and conception in high producer cows was 8 percent less than the others and also high milk yield was a risk factor for several reproductive disorders (Grohn and Rajala-Schultz, 2000). The objective of this study was to evaluate the effect of first 60-day cumulative milk yield on days open in Iranian Holstein dairy cows.