OBJECTIVE To assess postpartum sexual function in mothers using different infant feeding methods. METHODS The comparative cross-sectional study comprising women referred to health centres in Mashhad, Iran, was conducted from July 7 to December 11, 2011. Sexual function and infant-feeding method of the subjects were assessed four months after childbirth. Data was collected using the standard Female Sexual Function Index and baby's feeding method questionnaire. SPSS 11.5 was used for statistical analysis. RESULTS There were 366 women in the study with a mean age of 26.70±4.70 years who were using four methods of infant-feeding: exclusive breastfeeding, breastfeeding plus complementary feeding, formula milk, and breastfeeding plus formula. There was a significant difference between women's sexual function score and infant-feeding method (p=0.04). The highest score 6.23±3.5 belonged to women who had exclusive breastfeeding. CONCLUSIONS There was a difference in women's sexual function between different groups of infant feeding methods. The highest score of sexual function was found in breastfeeding women. Women need to be educated about exclusive breastfeeding and its positive effects on sexual function.