Homogeneously charged natural gas was burned in diesel engine by injecting a small amount of gas oil as an ignition source. The effects of the equivalence ration of natural gas, the injection amount and injection timing of gas oil, the intake temperature and the EGR rate, on ignition, the burninig rate of natural gas and the knock limit were investigated experimentally. The results show that natural gas starts to burn after ignition of gas oil, and the ignition delay is determined by the in-cylinder mean gas temperature at ignition timing, and the onset of knocking was almost dominated by the in-cylinder mean gas temperature which is around 1600K.