We checked the contents of 220 stomachs of specimens to assess effects of age and sex on diet composition of lizard Phrynocephalus grumgrzimailoi.We weighed each stomach,identified content component,counted individuals of each food item and weighed them,then calculated the index of relative importance (IRI) based on frequency of each food item appeared,percent of each mass weight,frequency of occurrence.We found out those stomachs contain large volume of arthropod and few vegetal items.Formicidae was the most important prey item of juveniles (female: IRI=95.91; male: IRI=69.04),while Coleoptera was the most important part in the diet composition of adults (female: IRI=35.49; male: IRI=32.58).Food niche breadth of adult female,adult male,juvenile female and juvenile male was 2.15,2.40,1.21 and 1.47,respectively.The food niche overlap between male and female juveniles was highest,whereas it was lowest between adult and juvenile male.The head of adult males was longer and wider comparing with adult female,and the food items of adult male were also with large volume than adult female.The lizard prey on different food items at different ontogeny stage.The phenomenon of adults of male and female fed on different size of prey might induced by sexual dimorphism in the head size and it could reduce the intraspecific competition.