The effects of ignition advance angle and excess air ratio on the combustion and emissions characteristics were investigated on a CNG engine at idle conditions. The throttle valve was fully closed. When the parameters were changed, the stepper motor was adjusted automatically to control the engine speed at 800r/min. When the θi is changed from 40 to 10o BTDC, the maximum ITE and the maximum power are observed at λ=1.1. As λ increases, the maximum ITE is corresponding to a greater θi. The COVimep will be increased if the θi is too advanced or too small. When λ is more than 1.3, the COVimep increases obviously. When λ=1.1, NOx emissions get the highest level.