The startup mechanism and power distribution of free piston Stirling engine (FPSE) are different from the traditional crank connecting Stirling engine. All the time, there is no paper to study the startup mechanism and power distribution of FPSE. In this paper, three necessary conditions of startup of FPSE have been first proposed. Theoretical analysis and numerical simulation have been used to illustrate the alpha, beta and gamma types FPSEs whether meet the startup conditions. Related experiments have been done to prove the theoretical analysis and numerical simulation on an alpha and a beta type FPSEs. According to the theoretical analysis, numerical simulation and experiments, some important results have been obtained. If a FPSE works stably, during a complete cycle, not only the total work in compression and expansion space should be positive, but also the work done by gas to both piston and displacer should be positive. To the alpha type FPSE, over a complete cycle the work done by gas to piston is negative and the work done by gas to displacer is positive. It does not meet the startup conditions. Therefore, the alpha type FPSE is impossible to startup. To the beta and gamma type FPSEs, over a complete cycle the work done by gas to displacer is positive. However, over a complete cycle the work done by gas to piston could be positive or negative. So it maybe meet the startup conditions of FPSE or not. So the beta and gamma type FPSEs could start up or not. Whether the beta and gamma type FPSEs could start up depends on the engine design and parameters configuration. (C) 2017 Elsevier Ltd. All rights reserved.