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Biomass (straw and wood) co-firing with coal was investigated in a drop tube furnace. Emission of NO and CO along the height of the furnace was monitored by a gas analyzer on line, and the content of unburned carbon (UBC) in fly ash was also tested. The effects of fuel type, temperature, co-firing and air staging ratio were considered. Results showed that along the height of the furnace, NO concentration increased first and then decreased s lowly for all the fuels, and NO emission from straw or wood combustion only accounted for about 1/3 or 1/2 of that from coal combustion respectively. In the present temperature range of 900-1100 o C, the increasing in temperature increased NO emission but decreased UBC in some degree, while the effect on CO emission was little. Under the condition of biomass co-firing, NO emission, CO emission and UBC were significantly reduced more with the increasing in biomass co-firing ratio. M oreover, results of air staging combustion showed that for coal combustion air staging notably reduced NO emission while increased UBC and CO emission, but for biomass combustion the effect of air staging was less significant. Further test of air staging on biomass co-firing indicated that only under the co-firing ratio of 0.4 could air staging play a great influence on NO emission and combustion efficiency. This present study could provide a guide for the optimization of biomass co-firing in industrial furnaces.