Effects of fuel reformation prior to load, on ignition timing in the HCCI process with DME (Dimethyl ether) has been studied by numerical integration of a DME detailed kinetic mechanism. HCHO was found to be most effective to retard ignition among major reformation products. By 6% addition of HCHO to DME, ignition timing is retarded by 25 degree CA. In addition, it is possible to estimate the effect to fuel consumption in cool flame stage, using simplified formula of low temperature oxidation which was applied for methanol addition in our former study. HCHO can be produced in fuel by keeping DME for 0.3 sec under 900K, avoiding the production of H_2 and CO.