Influence of internal heat leak on the optimal performance of a heat pump is examined in this paper. This is done by adding a heat leak term into an endoreversible heat pump. The relation between optimal heating load and coefficients of performance (COP) for a steady-state irreversible heat pump is derived. The performance characteristics of the irreversible heat pump is found to be deeply affected by the rate of heat leak. This model allows a more reasonable prediction in the performance of an actual heat pump.