The flow and heat transfer characteristics of regenerator materials were investigated in an oscillating flow not only for a conventional wire net but also for new materials such as sponge metal which can be set up easily in the Stirling engine. The prediction of pressure drop loss was possible by use of the hydraulic diameter as the representative length defined by the friction factor and Reynolds number. The regenerator efficiency and heat transfer coefficient were obtained by measuring the periodic temperature variations at both ends of the regenerators. A new relationship between regenerator reheat loss and regenerator size was also found from the analysis of the experimental data. The effects of the mesh diameter and the length of the regenerator on the indicated thermal efficiency were clarified.