In this paper, the performance of an irreversible Miller cycle is investigated. In the irreversible cycle, the linear relation between the specific heat of the working fluid and its temperature, the internal irreversibility described by using the compression and expansion efficiencies, the friction loss computed according to the mean velocity of the piston and the heat-transfer loss are considered. In addition to these irreversibilities, the effects of various design parameters, such as the minimum and maximum temperatures of the working fluid and the compression ratios on variation curves of the thermal efficiency versus the compression ratio, the power output versus the compression ratio and the power output versus the thermal efficiency are discussed. Nowadays, Miller cycle is widely used in the automotive industry and the results obtained in this paper will provide some theoretical guidance for the design optimization of the Miller cycle.