In this study, the procedures for the preliminary design of the turbo-shaft engine for the light multi-purpose helicopter are established. The engine specifications are determined through the performance analysis on the on-design and off-design conditions by the use of simulation program. In addition, the effect of humidity on the engine performance is examined by considering the change of the gas properties and characteristic maps due to moisture contents. The calculation results show that the engine power is reduced by the existence of moisture in working fluid.