Abstract Combustion behavior of mixed solid waste in a fluidized bed was studied. Bed, surface and freeboard temperatures were monitored continuously for that purpose. The effect of fluidization number and air factor on the combustion behavior of mixed solid waste of high moisture content in a fluidized bed combustor was investigated. Mixed solid waste was used as a sample which was formulated from four major waste compositions found in Malaysia namely food, paper, plastic and vegetable waste (green). Combustion study was carried out in a rectangular fluidized bed with sand of mean particle size of 0.34 mm as a fluidization medium. The range of fluidization numbers investigated was 3–11 U mf . The first part of the combustion study was carried out at stoichiometric condition. Results showed that the best fluidization number was 5 U mf in which the bed temperature was sustained in a much longer period. The second part of the study dealt with the effect of air factor on the combustion behavior conducted at 5 U mf . The chosen range of air factor was 0.4–1.2. It was revealed that the best air factor was in the range of 0.8–1. Energy balance was also performed to determine the thermal efficiency of the combustion. It was concluded that the thermal efficiency was significantly increased at higher air factor.