In order to extend the lean operation limit (LOL) of a spark ignition natural gas engine to further decrease its Nox emission, an experimental research was conducted to investigate the effect of hydrogen addition and various engine operating parameters on the LOL. The results show that hydrogen addition can significantly extend LOL and both the increase of engine load and intake air temperature result in greater (leaner) LOL. The effect of engine speed on LOL is not so straightforward. When fueled by pure CNG, LOL becomes greater with the increase of engine speed at low load, but while at high load, LOL drops if engine speed increases. However, when fuelled by hydrogen enrichment compressed natural gas (HCNG), engine speed has no obvious effect on LOL. Also an optimum spark advance to maximize LOL exists for a specific engine operating condition.  