Lean burn gas engines are expected to reduce NOx emission while improving engine performances such as output and thermal efficiency. Recently, an ignition method using a small quantity of diesel fuel (pilot fuel) as an ignition source for lean-burn gas engines has introduced further improvement of their performance. Generally, this method has been used for pre-chamber engines because it could not successfully lead to reduce NOx and Particulate emissions when adopted for open-chamber engines. However, the possibility of improvement of performances of open-chamber engines with this ignition method has also been expected(1). An experimental study was conducted to investigate the performance of an open-chamber gas engine with pilot fuel for ignition source. Experiments were conducted by using a single cylinder gas engine equipped with a common-rail injection system. Main gas fuel is supplied to the engine cylinder, and then a small quantity of diesel fuel (approximately 0.4∼1 % of total fuel energy input) was injected into the main chamber for ignition. This open-chamber gas engine has demonstrated superior performance, such as, a shaft-end thermal efficiency of 39.1 % with NOx level of 1.20 g/kW-h and 36.7 % with 0.40 g/kW-h, while maintaining COVimep within 2 %, which equals those of conventional spark ignited pre-chamber lean burn gas engines.