A study of reducing nitrogen oxide (NO) emission by using different fuel oil/hydrogen-rich refinery gas ratios (FO/RG) of the volumetric flow rate in a 130-ton/hr, full-scale, mediumpressure boiler was carried out. The influence of the FO/RG ratio on boiler outlet temperature, flue -gas flow rate and temperature, and O2 concentration in the excess air was also investigated. Test results show that by adjusting the FO/RG volumetric flow rate ratio from 5:1 to 1.8:1, fuel cost was reduced by as much as $33 million NT (~$1.1 million US) per year. Concentration of NO was lowered from 167 to 152 ppm, reducing NO emission a total of 72 tons per year. The air pollution fee was cut as much as $860,000 NT (~$27,000 US) each year. Hence, changing the FO/RG ratio of the volumetric flow rate can not only save fuel costs, but can also greatly reduce NO emission and air pollution fees. Results of this study can help to develop boilers that operate more economically and efficiently.