A liquid-piston steam engine is expected as a low cost and efficient energy conversion system at low operating temperatures (Yatsuzuka et al., 2011). In the present study, a numerical model of the liquid-piston steam engine is developed and a validation experiment is conducted to understand the mechanism of the oscillating steam engine, where a particular attention is paid to the steam accumulator volume. The numerical analysis is able to reproduce qualitatively the characteristics of the liquid-piston steam engine. In the case of the large steam accumulator volume, the indicated work and the indicated thermal efficiency decrease, because the accumulator contributes as a dead volume. On the other hand, indicated work is reduced for small steam accumulator volume cases, because negative work occurs. It is considered that the steam accumulator volume is one of the crucial design parameters for obtaining efficient liquid piston steam engine. (C) 2015 Elsevier Ltd. All rights reserved.