The inplane free vibration frequencies and nonlinear internal resonance of circular arches were studied. Based on the assumptive vibration function, the nonlinear equations were discretized by the Galerkin method. First, the frequencies of derived system were obtained, thus the effect of different parameters on frequencies were studied. Then, the method of multiple scales was applied to solve the equations. The results showed that the radial frequency was greatly affected by the height of the crosssection, but the tangential frequency wasn't. Three types of internal resonance form may be activated. The further studies of the equations in this paper pointed out that internal resonance was determined by the boundary conditions.