Eastern Rock Nuthatch (Sitta tephronota) and Western Rock Nuthatch (S. neumayer) are two closely related and sympatric species in Iran, which have been known as the classical case of character displacement since 1950. Nevertheless, some questions related their niche partitioning is still unanswered. This study was conducted to determine the diet differentiation, as an important niche dimension, between the two species in their sympatric zone along the Zagros Mountains. The dietary composition of the two species was studied by analyzing stomach contents, using a stereo-microscope. The result showed that the most diet of the two species in spring was insects. Eastern Rock Nuthatch fed predominantly on beetles (Coleoptera), bugs (Hemiptera), and grasshoppers (Orthoptera) whereas Western Rock Nuthatch consumed mostly on bugs (Hemiptera), beetles (Coleoptera), and butterfly’s (Lepidoptera). We used analysis of covariance for finding significant differences in 2 measurement traits in bugs and beetles .Covariance analysis revealed that the relationships between body and prey size differ between the two species only in one measurement (length of head in bugs), as Eastern Rock Nuthatch captured larger prey size. Furthermore our result demonstrated that the niches breathe of these species is rather equal, as well as their niche overlap is high. Overall our results confirmed the importance prey size in trophic niche partitioning and possibility of coexistence for closely related species.