The engine performance, combustion characteristics and the effect of injection timing differences of a prechamber-type diesel engine operated by Diesel-fuel and methanol were investigated. When the Diesel-fuel is used as a pilot fuel, the ignition lag was mainly controlled by the Diesel-fuel.The maximum thermal efficiency occurs with the optimum main methanol injection timing. The optimum methanol injection timing is about 2 to 3 degrees crank angle before the ignition, as methanol vapor is required for smooth combustion. This was determined from results showing that the methanol combustion was initiated by flame propagation from the combustion of Diesel-fuel. It is advantageous that parts of the Diesel-fuel and methanol sprays be mixed.