To obtain fundamental information on the combustion and emission characteristics of refuse derived fuel (RDF) in fluidized bed combustors, the behavior of NOx emission was investigated with a special focus on the effects of CaO and Cl on it. Several model RDFs, in which CaO and Cl were present or not, prepared from the mixture of news paper, bean-curd refuse and PVC. The molecular composition of CaO, Cl, C, H and N in the model RDFs was adjusted to a composition almost the same as those of commercial RDFs prepared from municipal solid wastes. The model RDF containing both CaO and Cl had higher conversion of fuel-N to NOx than that of the model RDF containing only either one of the two. The conversion of fuel-N to NOx of model RDF containing either CaO or Cl did not differ from that of model RDF with no CaO nor Cl. The effect of CaO-Cl coexistence on NOx emission increase was intensified either when stoichiometric air ratio was decreased or CaO content was increased. The catalytic activity of CaO for NHl oxidation to NOx was found to be much increased by the existence of Cl in RDF. Furthermore, it was confirmed that preparation process, the density and shape of RDF and the difference of Ca form (i.e. CaO or Ca (OH)(2)) are the minor factors concerning the conversion of fuel-N in RDF to NOx.