The increasing demand of energy and environmental problem due to the consumption of fossil fuel has led the exploration of biomass fuel as an alternative energy. One of the potential biomass feedstocks that is available in Malaysia is Domestic Wastewater Sludge (DWS). In this study, DWS fuel characterization and the effect of experimental condition such as gaseous atmosphere, heating rate and particle size to the thermal behaviour of DWS has been carried out. In addition, kinetic parameter such as activation energy was determined by using the iso-conversional model free kinetic method, Flynn Wall Ozawa (FWO) model. Furthermore, lower heating rate and smaller particle size of DWS produced more volatile gas (84.51), resulted to low ignition temperature (209°C) and low burnout temperature (515°C). Higher volatile indicates that it can accelerate the ignition process. In addition, low ignition temperature and low burnout temperature reveal that DWS can be ignited easily and shorten the time for combustion process. Based on the finding, the average activation energy for combustion of DWS which is about 143.86 kJ/mol. © 2020 Published under licence by IOP Publishing Ltd.