Research and development of an advanced air/fuel ratio control system for stationary lean-burn gas engines for co-generation have been carried out. The unique feature of this control system is that the air/fuel ratio is controlled by the feedback from the fluctuation of engine speed. Air/fuel ratio is controlled as high as possible to achieve low NOx emission while maintaining the fluctuation of engine speed within its maximum tolerance. Experiments have been conducted by using a test engine equipped with the new control system. Superior performance of the system has been demonstrated.