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Objective:Human milk (HM) is the most necessary and complete food for infants and their survival. It is a dynamic system influenced by different factors such as the sex of the infant. The study of the factors affecting the composition of mother's milk can provide us better insight into the nutritional needs of infants with different sex and improve the nutrition of babies who do not have access to HM. This study aimed to study the effects of infant sex on mother's milk composition.Methods:Sixty-one mother's milk samples were collected from mothers with female or male infants and divided into two (32 sons, 29 daughters) groups. The samples were analyzed by the Lacto-Scan milk analyzer and the obtained data were compared between the groups.Results:The results showed that the mean fat content in the milk samples of mothers with female infants (daughters group) (3.42 ± 1.80%) was significantly (p= 0.029) higher than that in mothers with male infants (sons group) (2.53 ± 1.18%). However, the sons group had higher (p= 0.024) levels of salts (0.76 ± 0.14%) compared with the daughters group (0.67 ± 0.18%). There was no significant (p> 0.05) difference in other indices between the groups.Conclusions:The sex of the infant is one of the important factors affecting the composition of mother's milk. The difference in the composition of mother's milk may reflect the differences in metabolic substrate needed for optimal growth and development in female and male infants.