We present theoretical calculations of the line shapes of emission spectra of ZnO micro spheres (MSs), including the exciton-polariton and Purcell effect. Our calculation explains the red shift of emission peaks of whispering gallery modes (WGMs) in UV range commonly observed in ZnO MSs. We show that the red shift of the UV emission peak is caused by the combination of cavity effect and the polariton dispersion. The positions and relative strengths of sharp peaks arising from WGMs are also simulated by our calculation, and theoretical predictions match well with experimental data. Our calculation provides useful guide lines for the design of MS cavities for applications in white-light illumination, optical communication, and biosensing.