This study investigates influence of fuel injection and ambient gas conditions on lean burn combustion process ignited with diesel fuel. Fuel injection pressure, ambient gas pressure and equivalence ratio were varied in a wide range and influence of those parameters was closely evaluated based on the high speed imaging and the pressure analysis. The result showed that the ignition delay period of diesel fuel was prolonged at high injection pressure and low ambient gas pressure. The initial combustion period was decreased with an increase of the ambient gas pressure when the injection pressure is high.