This study conducts a series of numerical simulation to investigate systematically the improvement of power as well as thermal efficiency of the Miller cycle gas engines. Novel approach of the numerical simulation was to develop full chemical-kinetics methods to predict knocking phenomena for practical use. The method was successfully incorporated into the one-dimensional cycle simulator of the TSpark code developed by UMIST. The study predicted the performance of the Miller cycle, with particular attention to the improvement of thermal efficiency with higher expansion ratio and of engine power with lower compression ratio. Results show the Miller cycle substantially improves either efficiency or power by optimizing the compression and expansion ratios. Trade-off relation between the improved power and efficiency is evaluated.