Co-firing of biomass and coal has drawn many attentions because it can reduce the amount of CO2 release of the coal power plant/incinerator. However, the higher amount of sulfur, chlorine, potassium and calcium in biomass could lead to a more serious corrosion. In this study, the high-temperature corrosion of Fe-Cr-Mo alloy at 400 °C to 600 °C in co-firing of biomass and coal was performed. The results show that the weight change of specimen tested in co-firing of biomass and coal is 1.7 to 2.4 times of that for specimen tested in dry air. Also, the weight change of specimen tested in co-firing increased with the tested temperature and time increased. For the specimen tested in co-firing with different ratio of biomass/coal, the weight change of specimen increased with the ratio of biomass/coal increased.