In the thermoacoustic theory, thermoacoustic phenomena are discussed in terms of both state quantities and flow quantities such as heat flow and work flow. Since the Stirling refrigerator is regarded as one of thermoacoustic devices, a simulation code THERMOACOUSTICA based on the thermoacoustic theory was used in order to examine the frequency dependence of performance of a Stirling refrigerator. Results of the simulation qualitatively agreed with the prediction disregarded the frequency dependence of temperature distribution of the regenerator. When the simulation results were examined in detail, it was, however, necessary to consider the frequency dependence of temperature distribution of the regenerator. Operation frequency in which the COP of the refrigerator becomes highest is considerably lower than that in which a refrigeration capacity becomes largest. Therefore, criterion of selecting the optimum condition is one of problems in designing a regenerative refrigerator.