This study examined the effects of fuel composition and intake pressure on two-stage high temperature heat release characteristics of a Homogeneous Charge Compression Ignition (HCCI) engine. Light emission and absorption spectroscopic measurement techniques were used to investigate the combustion behavior in detail. Chemical kinetic simulations were also conducted to analyze the reaction mechanisms in detail. Blended fuels of dimethyl ether (DME) and methane were used in the experiments. It was found that the use of such fuel blends together with a suitable intake air flow rate corresponding to the total injected heat value gave rise to two-stage heat release behavior of the hot flame, which had the effect of moderating combustion. The results of the spectroscopic measurements and the chemical kinetic simulations revealed that the main reaction of the first stage of the hot flame heat release was one that produced CO from HCHO. The results also indicated that the main reactions of the second stage of the hot flame heat release were ones that produced the final products of CO₂ and H₂O from CO and HO₂. It was observed that DME was consumed by the cool flame and in the first stage of the hot-flame heat release, whereas methane was mainly consumed in the second stage. That explains why the heat release of the hot flame occurred in two distinct stages.