BACKGROUND: Affecting the female reproductive organ and the layers around the oocyst, some ration ions could facilitate the penetration of sperm with specific chromosome type and change the sex ratio in mammal infants. OBJECTIVES: The purpose of the present study was to evaluate the effect of potassium supplement on the sex ratio in rat. METHODS: The percentage of moisture, ash, crude fat and protein, sodium, potassium, calcium, and energy was measured in the ration of rats. In one gestation period, the male and female pups from 32 female and 8 male adult rats were counted as control group. In experimental group, potassium citrate (36% potassium) was used in control group ration to raise the level of potassium from 0.35% to 0.8% and 12 female and 3 male adult rats were fed with that ration in two gestation periods. The number of male and female pups in both groups was counted at the age of three days. Sex ratio in both groups was compared using chi-square test (SPSS, Ver. 11.5). RESULTS: The ration of control group had 90% dry matter, 8% ash, 4.1% crude fat, 21.6% crude protein, 70.8% total digestible nutrients (TDN), 0.4% calcium, 0.3% potassium and 0.1% sodium. The ratio of male pups in the rats were fed with potassium supplement (1.52, 100 male and 66 female pups) was higher (p=0.03) compared to the control group (0.94, 115 male and 122 female pups). CONLUSIONS: Raising the level of potassium in food ration can increase the number of male pups over 10% in each gestation.