The dirt patterns and contents in burning heavy oil is analysed. When burningheavy oil with rather high viscosity, the great majority of dirt comes from remainder of rem-nant carbon and coke-shell formed by colloidal asphalt of heavy oil in furnace. The dirt con-sists of dust and soot. In oil -burning boiler using high viscosity heavy oil with great atomizedparticle sise of oil spray, the mixing of oil spray and air is non-uniform and the dirt ofexhausted gas can exceed the emission standard of air pollutants. The basic method ofreducing dirt of heavy oil-burning boiler is to improve the quality of oil atomization andcombustion in furnace.