We present a theoretical analysis of a high-T c superconducting microstrip resonator using spectral domain method. Fourier transformed impedance Green's functions are derived and the resonant frequencies are calculated for three different substrates, as a function of the resonator length, in Galerkin's procedure. Effects of thermal expansion of the substrate and surface impedance of the superconducting film are incorporated in the theory to analyze the temperature dependence of both the unloaded Q and the resonant frequency. The effects of these two mechanisms are analyzed separately and compared. It has been found that the thermal expansion of the substrate is mainly responsible for linear shift in the resonant frequency below 0.8 T c . The effect of surface impedance is found to be dominant only in a temperature range close to the transition temperature for shift in both resonant frequency and Q.