Using natural gas as a clean fuel is raising. In many industrial combustion systems, like industrial furnace and boilers, a large portion of heat transfer is done by radiation and using natural gas in these industrial systems leads to decrease in radiation heat transfer that faces the Manufacturers with many problems. The addition of solid reactant particles into the flame is one of the attractive methods that are considered by many researchers to increase the radiation heat transfer in non-luminous flame such as hydrogen and natural gas flame. In this study, the effect of coal particles injection into the natural gas diffusion flame on flame structure, radiation heat transfer, temperature profile, and thermal efficiency has been considered. The results show that the injection of coal particles into the natural gas by increasing the solid soot particles in the flame structure, increases the reaction surface and flame luminosity and with increasing the radiation emissivity coefficient, increases the radiation heat transfer and thermal efficiency 43% and 21% respectively. Whereas change in flame temperature is very low and is 47˚C in its extreme limit.