Heat transfer enhancement of heaters in Stirling engines plays an important role in improvement of the thermal efficiency of the engines. Effective use of wall radiation in heaters working at high temperatures enhances the heat transfer between combustion gas and heater tubes. In this paper, we report the results of numerical calculation of the improvement in heater performance by the heat transfer enhancement, as well as also the resultant improvement in engine efficiency. Heat transfer calculation is coupled with the engine analysis incorporating detailed key phenomena. As a result, the engine efficiency is shown to be improved more than 10 % by the effective use of radiation in the heater, and the effects of air excess ratio, temperature efficiency of the preheater, maximum tube temperature etc. are also discussed.