Because blending hydrogen with natural gas can allow the mixture to burn leaner, reducing the emission of nitrogen oxide (NO(x)), hydrogen blended with natural gas (HCNG) is a viable alternative to pure fossil fuels because of the effective reduction in total pollutant emissions and the increased engine efficiency. In this research, the performance and emission characteristics of an 11-L heavy duty lean burn engine using HCNG were examined, and an optimization strategy for the control of excess air ratio and of spark advance timing was assessed, in consideration of combustion stability. The thermal efficiency increased with the hydrogen addition, allowing stable combustion under leaner operating conditions. The efficiency of NO(x) reduction is closely related to the excess air ratio of the mixture and to the spark advance timing. With the optimization of excess air ratio and spark advance timing, HCNG can effectively reduce NO(x) as much as 80%. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.