Thermal effect on phase masks in wavefrontcoded athermalized imaging systems is analyzed. Several wellknown oddsymmetric phase functions at different temperature are derived. The properties of phase functions suffering from thermal effect are presented. The performances of the phase functions are evaluated with and without considering thermal effect by numerical simulation. The results show that the similarity of outoffocus MTFs and recoverability of blurred encoding images of the wavefrontcoded athermalized system are degraded severely by the thermal effect.