Abstract: The ash deposition characteristics during co-combustion of coal and sawdust was studied in a 50 kW furnace with an online monitoring system composed of CCD camera and ash deposition probe. The deposition process can be divided into three stages: slow growth stage, rapid and stable stage. The stable thickness of bituminous coal increases with increasing proportion of sawdust, while that of lean coal is opposite. The stable thickness of bituminous coal are 1.37, 3.85, 11.50, 20.56 mm and the stable relative heat flux are 0.44, 0.41, 0.30, 0.26 when blending ratio of sawdust are 0, 6.7%, 15%, 22% respectively. The stable slags thickness of lean coal are 18.65, 10.97, 9.78 mm and the stable relative heat flux densities are 0.29, 0.31, 0.33 when blending ratio of sawdust are 6.7%, 15% and 22%. Ca and K in the initial layer of deposition significantly increases in the co-combustion. The melting fraction calculated by FactSage software shows that at the same tem-perature, the melting fraction of ash increases as the sawdust is increased. Because the wood ash contains more alkali metal oxides such as Na₂O and K₂O, while the content of Al₂O₃ and SiO₂ is less, which lowers the melting temperature of ash.