Vapor compression and condensation system (VCC) can dehydrate wet wastes such as biomass with high efficiency, using same principle as heat pump. However, mass flow of treated steam decreases as time goes on. The causes of this problem are the lack of heat transfer area, the sediment of solid materials in the evaporator on the heating surface and the existence of non-condensable gas in the condenser. In addition, excessive superheated vapor causes breakdown of compressor or scorching of biomass. Thus, we constructed the VCC simulation model in order to investigate the effect of the factor obstructing heat transfer and the water injection at the inlet of compressor on overall system performance. Also, the method to select optimal design parameters is proposed for the improvement of machine performance or downsizing. In this paper, we defined objective function, in which weighting factor is applied to the size of heat exchanger and the input power to VCC, and specified design parameters to minimize the objective function. Then, we performed parametric analysis to investigate the effect of mass flow of water injection and the heat transfer coefficient on the objective function, under the different weighting factor. As the result, it was concluded that the discharging means of heat transfer obstructive factor with high capability was necessary for optimal design of VCC.