Abstract: The objective of this retrospective study was to investigate the potential factors affecting sex ratio on dairy herds in East Azarbaijan province of Iran. The data used in this experiment were collected from 186 registered dairy herds are routinely compiled on a monthly basis (from June 1990 to July 2007). For statistical analyses, sex ratios were compared with expected value using goodness-of-fit Chi-square ( χ2) analysis. The effect of some factors such as herd size, artifical insemination year and kind of frozen semen (internal and external ) on sex ratio was determined. Statistical analysis of data shown that herd size had significant effect on sex ratio (χ2 = 5.989, df = 4, p < 0.05). The χ2-test for year variables were significant (χ2 = 52.496, df = 17, p < 0.01), Also the effect of frozen semen (internal and external) on sex ratio was not significant (χ2 = 5.893, df = 7, p > 0.05). Results suggested a positive effect of herd size and year on sex ratio therefor it is possible using this result as a practical and inexpensive method of sex ratio modification.