Research and development of a low NOx lean burn combustion for open chamber gas engines have been carried out. The target NOx level has been set below 200 ppm with superior thermal efficiency to the engines with three-way-catalyst. A number of open combustion chambers were tested. Swirl combustion chambers NEBULA and TG (the latter was newly developed by the authors) and a squish chamber REENTRANT have satisfied the target while maintaining sufficient stability of combustion. It has been demonstrated that open chamber gas engines are potentially capable of achieving low NOx emission while maintaining high thermal efficiency.