An experimental investigation on effects of hydrogen fractions (0%similar to 11.09%) on lean burn combustion and emission characteristics of SI gasoline engine was carried out on a modified premixed gasoline engine equipped with a hydrogen direct-injection system, under the condition of the excessive air coefficient from 1.0 to 1.5. With the increasing hydrogen addition fraction, the combustion speed increases, mean effective pressure and thermal efficiency are improved. Moreover, flame development and propagation duration are shortened, the peak cylinder pressure increases and its corresponding crank angle advances, the maximum rate of heat release, both the maximum mean gas temperature and the maximum rate of pressure rise increase, while HC and CO emissions decrease and NOx emissions increase. Hydrogen direct injection does not reduce volumetric efficiency, thus engine power performance improves. The addition of hydrogen makes stable combustion at 1.5 excessive air coefficient possible, and the effective thermal efficiency increases, along with a significant reduction in CO and NOx. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.