This paper investigates the effect of high hydrogen volumetric ratio of 55% on performance and emission characteristics in a turbocharged lean burn natural gas engine. The experimental data was conducted under various operating conditions including different spark timing, excess air ratio (lambda), and manifold pressure. It is found that the addition of hydrogen at a high volumetric ratio could significantly extend the lean burn limit, improve the engine lean burn ability, decrease burn duration, and yield higher thermal efficiency. The CO, CH(4) emissions were reduced and NO(x) emission could be kept an acceptable low level with high hydrogen content under lean burn conditions when ignition timing were optimized. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.