Allometry is the study of the size of different somatic/sexual morphological trait related to the whole body size of an insect and expressed as an allometric slope. Allometry provides an effective method for study of the effects of selective pressures on the morphology and performance of different body parts. For example, male genitalia in the insects often shows negative allometry in response to natural selection. Studies now targeting to identify the ecological and evolutionary conditions associated with different patterns of allometry and phenotypic variation within insects to understand how natural selection shape animal morphologies. In the present study, effect of larval diet on the allometric relationships of adults in the grain moth, Sitotroga cerealella (Lepidoptera: Gelechiidae) is studied. Larvae were reared on three different diets including two barley and one corn varieties. 40 emerged adults were selected randomly from each treatment and male genital traits including uncus length and width, valval length and width, length of valval spine and length of median part of armature and somatic traits including forewing length and width, length of femur, tibia and tarsus of hind leg and length of first and second spine of hind tibia, were measured and analyzed. We used principal component analysis (PCA) to calculate the index of body size by different somatic traits of each individual. According to the results, diet quality had significant effects on the body size of adult males and females. Although, all male genital traits in three diets showed negative allometry in related to the whole body size with allometric slopes significantly less than somatic traits. Dispersion around the mean (CV) of studied body parts have no significant differences within three diets. These results indicate that different larval diets have significant effects on the body size of this species and largest body were on corn and least body size were on barley var. native. However, different larval diets have no significant effect on the allometric slopes of the studied body parts and their selective role and all allometric slopes are consistent with one-size-fits-all hypothesis.