The technology of co-firing was used to study the combustion of groundnut shell and coal at three different ratio of biomass (10%, 20%, and 30% in weight) and combustion of miscanthus and coal at three different ratio of biomass(10%,20%,30%) using thermo gravimetric analysis (TGA). When compared to firing of coal, the co-firing of coal and biomass (groundnut shell, miscanthus) has shown a good combustion result. The ignition temperature (Ti) burnout temperature (Tf) is reduced in the co-combustion when compared to the combustion of coal alone. The best blend ratio of coal/groundnut shell is 80%/20%. The best blend ratio of coal/miscanthus is 70%/30%.