Oxy-steam combustion behaviour of rice straw (RS) pellets was studied at high temperatures experimentally in a new plasma combustion system and numerically using an upgraded one-dimensional unsteady-state single particle combustion model with a new homogeneous devolatilization volatile combustion and heterogeneous char oxidation sub-models under different O2/H2O/CO2/N2 gas compositions. Experimental results showed that the temperature gradient profiles for both the surface and center decreased significantly when the steam was added to the gas mixture at 20 and 40%. At 30% H2O, the heat consumption of the gasification reaction decreased which resulted in a higher particle temperature and a higher char oxidation reaction rate. Combustion characteristics for heating, ignition, devolatization, and combustion stages were obtained and confirmed through the time of recorded pictures by high-speed camera. The obtained experimental results were validated through fitting the measured values of surface and center temperature-time profiles, ignition time, final pellet diameter, and time for the maximum amount of released volatiles, under different H2O and CO2 concentrations, with those predicted from the model solution. Also, the mass of the released volatiles and pellet diameter history were predicted from the model solution. Results of the model solution proved the reality and credibility of the measured and calculated experimental findings.