In this study, the combustion characteristics of firing blast furnace gas (BFG) individually or in combination with coke oven gas (COG) were investigated theoretically and experimentally. The theoretical results showed that at the same heat release rate under the operating conditions of stoichiometry, firing BFG individually had a greater amount of fuel gas volume flow rate, a smaller amount of air volume flow rate, and a lower adiabatic flame temperature than firing BFG with support fuel without preheating. Meanwhile, the volume flow rate of flue gas for the former was greater than the latter. The experimental results obtained from a co-firing of BFG and COG fueled-boiler in a steel company showed that whether at a higher or lower boiler load, with gradually decreasing the amount of COG supply, flame instability (lift-off and blow-out) did not occur and the rate of CO emission was very low. In addition, a decrease in the amount of COG supply brought about a great benefit to the reductions of both CO2 and NOx emissions.