The objectives of the present study is to clarify ignition characteristics, the combustion process, the knock limit and the misfire limit of natural gas mixed with a small amount of dimethyl ether (DME) in a HCCI engine. In the combustion test, natural gas and a small amount of DME were charged into the suction air homogeneously. The equivalence ratio of natural gas was increased to find the knock limit or the misfire limit of the HCCI test engine under a constant DME amount. The effect of the natural gas addition on suppression of the low temperature reaction of DME, and the effects of the DME amount and the intake temperature on the reaction rates, the knock limit of the DME/natural gas mixture, and the operation load range of the HCCI engine were investigated experimentally.