To develop a highly efficient CFB generating process, basic RDF combustion properties are observed by the thermal balance technique for two RDF produced by different processes. It was observed that RDF weight was reduced in two stages with exothermic reaction in their combustion. This result indicates that these RDF consist of two kinds of combustibles, one burning at low temperature and another at high temperature. From the small weight difference of RDF sample between after combustion and after pyrolysis, it was found that combustion is controlled mainly by gas-phase combustion of volatile matter released by pyrolysis. Furthermore, it was also found that the first stage reaction of each RDF has the same reaction mechanism, however the reaction mechanism of the second stage depends on the RDF production process.