The Miller cycle was applied to a turbo-charged 324 kW gas engine with three-way catalyst. Three different methods of the Miller cycle were applied, the Early Rotary-Valve Closing (ERVC), Late Intake-Valve Closing (LIVC) and a combination of the ERVC and LIVC methods. The effect of the methods on engine performance was extensively investigated. The experimental results demonstrate a promising performance of the LIVC method, which substantially improves the thermal efficiency up to 38%, compared to 34% by the conventional Otto Cycle. The LIVC method is expected to improve engine performance with inexpensive modification. The combination of the ERVC and LIVC methods appears to further improve thermal efficiency.