This article presents basic results of the design analysis of a combined gas-vapour micro power plant with a waste-heat boiler without exhaust reheat, working in the vapour section in the organic rankine cycle (ORC) system. The analysis concerned working media most frequently used in those types of power plants and that plan to be used in the future. The object of the analysis was a micro power plant with a 30 kW gas (air) turbine for which the low-boiling medium was selected in such a way as to obtain maximum possible power output of the vapour turbine and, consequently, the highest efficiency of the combined cycle. The analysis also included vapour micro power plants with heat regeneration for so-called dry media and cycles without heat regeneration for so-called wet media. The amount of thermal energy available for utilization in the cogeneration micro power plant at the assumed condenser temperatures equalling 95ºC, 55ºC, and 30ºC, respectively, was assessed. The most favourable medium in terms of the obtained power output and efficiency of the combined cycle was selected and the effect of the low-boiling medium on design parameters of the vapour micro turbine was assessed.