The effect of co-firing of biomass fuels with oil shale on combustion was investigated Thermogravimetric analysis and differential scanning calorimetry were the tools used to perform the investigation. Since the combustion of biomass is highly exothermic, biomass fuels can serve as an appropriate fuel feedstock. Biomass fuels producing much volatile matter and containing less cellulose are good candidates for co-firing with oil shale. The biomass samples used in the study were hazelnut shell, wheat bran, poplar, and miscanthus. Co-firing of biomass/oil shale blends was performed using different biomass ratios (10, 20 and 50% by weight).