A theoretical model for predicting thermodynamic behavior of thermal-lag Stirling engine is presented in this study. Without a displacer and its link, the thermal-lag engine contains only a moving part (piston) and a static part (regenerative heater) in engine's cylinder and hence, is regarded as a unique type of Stirling engines that featuring rather simple mechanical structure. In this study, a numerical simulation of thermodynamic behavior of the thermal-lag Stirling engine is performed based on the theoretical model developed. Transient variations of temperatures, pressures, pressure difference, and working fluid masses in the individual working spaces of the engine are predicted. Dependence of indicated power and thermal efficiency on engine speed has been investigated. Then, optimal engine speeds at which the engine may reach its maximum power output and/or maximum thermal efficiency is determined. Furthermore, effects of geometrical and operating parameters, such as heating and cooling temperatures, volumes of the chambers, thermal resistances, stroke of piston, and bore size on indicated power output and thermal efficiency are also evaluated. (C) 2012 Elsevier Ltd. All rights reserved.