The present work deals with the analysis of the application of a single stage partial admission axial expander in organic Rankine cycle, in order to recover the heat rejected by an internal combustion engine. The selected fluid is R245fa, whose real gas behavior is relevant under the studied conditions, as modeled by the Redlich-Kwong-Soave equation of state. A loss model for the flow through the axial blades is proposed in this work, based on the boundary layer theory, the concept of diffusion factor, wind tunnel cascade tests available in literature and the conservation equations for compressible flow. The diffusion factor is the parameter responsible to quantify the adverse pressure gradient on the blade profile surfaces. The basic geometry and dimensions needed to achieve maximum expander efficiency are determined under several subcritical and supercritical cycle conditions by means of a restrained design optimization algorithm. The optimum value for the evaporator temperature under subcritical cycle is stablished so as to obtain the maximum power from the recovery cycle, according the constructive alternatives considered. The single stage expander design is shown to be greatly influenced by media compressibility and the use of convergent-divergent profiled nozzles is promising to achieve the highest performance potential, especially at high evaporator pressure conditions.