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The effects of Miller Cycle on a lean-burn natural gas engine performance are studied using a GTPower model.Miller cycle is based on changing the intake valve closing time to achieve high expansion ratio than compression ratio,therefore it can improve the thermal efficiency and fuel economy to some extent.In this paper,some engine parameters such as increased compression ratio and late intake valve closing time are considered in effort to improve engine perfomance.The results showed that the power output is slightly decreased due to the back flow of the charge,the boost pressure and ignition timing can be modified to make up the power.Intake valve closing timing plays an important role in improving thermal efficiency and decreasing specific fuel consumption.As a result,the thermal efficiency and the specific fuel consumption could be improved by 2.52% and 2.46% respectively.