The combustion characteristics of co-firing bio-oil produced from the fast pyrolysis process of coffee bean residue and diesel in a 300-kWth oil-fired furnace are investigated. Using bio-oil to completely replace fossil fuels has limitations since bio-oil has undesirable properties, such as high water and oxygen contents, high viscosity, and low heating value. However, a low blend ratio of bio-oil used as a substitute for petroleum-derived oil has advantages; i.e., it can be easily combusted in existing furnaces without modifications. Thus, a promising solution is the partial substitution of diesel with bio-oil, rather than completely replacing it. A furnace test is performed for diesel alone and bio-oil/diesel blends with 5 vol % bio-oil. The results show that excellent stable combustion is observed during the co-firing test. Compared with diesel, with 5 vol % bio-oil content in the blends, both the wall temperature and gas temperature drop only slightly and exhibit similar furnace temperature distribution; meanwhile, comparable NO emissions (smaller than 57 ppm) are obtained, and lower CO2 emissions are achieved because biomass is both carbon neutral and renewable. Moreover, SO2 and CO emissions under these two burning conditions are very low; SO2 and CO emissions are smaller than 6 and 35 ppm, respectively.