A simulation code has been developed for analyzing plasma behavior and microwave propagation in a microwave ion engine. The code consists of PIC (particle-in-cell) method and FDTD (finite-difference-time-domain) method. In the present study, 10-cm-class microwave ion engine developed by the Japan Aerospace Exploration Agency (JAXA) is adopted as a calculation model. The microwave propagations in the discharge chamber are compared under boundary conditions such as absorption and reflection. In the calculation with plasma, effect of boundary condition on electron energy is investigated. As a result, it is found that increase of electron energy is larger for reflection boundary condition than absorption one. (c) 2005 Elsevier B.V. All rights reserved.