This communication presents the Second Law analysis based on the exergy concept for the performance of Simple and Regenerative-Reheat Rankine heat engines generally used in Solar Thermal Power Systems. Basic energy and exergy analysis for the heat engine system components are carried out for evaluating the energy and exergy losses as well as the exergetic efficiencies for both the Simple and Regenerative-Reheat Rankine heat engines. Relevant energy and exergy flow diagrams are drawn to show the various thermodynamic and thermal losses. It is found from the results that most of the exergy losses occur in the boiler and these losses are significantly reduced by the incorporation of a regenerator and reheater, whereas the results of the energy analysis shows that the main energy loss takes place in the condenser. This is because in the boiler high quality energy is lost but in the condenser low quality energy is lost. The incorporation of the regenerator and reheater results in a reduction of the total irreversibility rate of the cycle by 28.77%. The corresponding improvement in the exergetic efficiency is 16.42%.