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The effects of various injection timing of methanol on thermo-atmosphere combustion of methanol by port injection of dimethyl ether (DME) and direct injection of methanol were experimentally investigated. The experiment results show that, as injection timing is at 6 degree before TDC, the combustion process comprises three stages: low temperature heat release of DME, high temperature heat release of DME and diffusion combustion of methanol. As injection timing increases, premixed combustion proportion of methanol is increased and diffusion combustion proportion is decreased. As injection timing increases to 126 degree before TDC, diffusion combustion of methanol disappears. At this time, the combustion process shows typical two stages heat release of HCCI combustion. As injection timing increases, required DME rate is increased, combustion efficiency and indicated thermal efficiency all first increase and then decrease.