This paper reports thermodynamic optimisations based on the maximum net cycle work and the maximum thermal efficiency criteria for an irreversible single and double reheat Rankine cycle, which includes the internal irreversibilities resulting from the adiabatic processes. The study encompasses the effects of boiler pressure and temperature on the general and optimal performances of the plant in terms of net work output and thermal efficiency. Therefore, the net work output and thermal efficiency of the plant are obtained by introducing the mechanical, boiler and isentropic efficiencies. The effects of boiler pressure and temperature on the optimal reheat pressures of the single and double reheat Rankine cycles are also investigated. It is shown that for the theoretical Rankine cycle, the optimal reheat pressures of single and double reheat Rankine cycle are both fairly affected by the boiler pressure and temperature.