The generation of mechanical power had been a challenge for mechanical engineers and a great deal of attention has been focused on designing system for power generation. Thermal power cycles are important devices used in practical engineering application for thermal power generation and control of environment. Several thermodynamic approaches are possible to analyze the performance of these cycles. The most traditional approach is the first law of thermodynamics or energy balance analysis; however, this concerned only with the conversion of energy, and therefore it can not be show how or where irreversibility in a system or process occur. Second law analysis is another well-known method used to analyze thermodynamic cycles, but to some, it may be still new. Unlike first law analysis, second law analysis determines the magnitude of irreversible processes in a system, and thereby provides an indicator that the points the direction in which the engineers should concentrate their efforts to improve the performance of thermodynamic systems. This thesis is an attempt to physically understand and evaluate the internal irreversibilities due to heat transfer and fluid flow in various thermal power cycle used for power generation. This thesis present investigations on thermodynamic modelling and effects of irreversibilities in thermal power cycles using the concept of exrgy analysis and entropy generation. Detail literature review on second law analysis of thermal power cycles is reported in Section I.0 First and second law analysis of stem power cycle, gas turbine cycle, combine Brayton/Rankine power cycle, indirect fired air turbine multi stage…. have been carried out. Analytical expressions for the performance parameters involving the relevant variables for these systems under given conditions, corresponding to available power output and exergy destruction in the system, have been obtained. Detailed parametric study has been done and result are presented. This study include the effect of pressure ratio, cycle temperature ratio, number of reheat stages, pressutre drop in heat transfer devices, and the refrigeration temperature on the performance parameters of the thermal power cycles. Second law based performance assessment of regenerative-reheat steam power cycle has been carried out in terms of irreversibility analysis. The reduction in irreversible losses with the addition of backward, cascade type feed water heater and a reheat options are compared with a conventional first-law assessment. The second law indicate that the maximum exergy is destroyed in the boiler and that these thermodynamic losses are significantly reduced by incorporating feed water heater. The incorporation of feed water heating results in a reduction of total irreversility rate of the plant by 18%. The corresponding improvement in thermal efficiencies is 12%. These two values are enhanced to 24% and 14% respectively, by the incorporation of reheat in addition to regeneration.