To find the best combustion conditions in the heavy oil burner kinetic viscosity of heavy oil A, B and C at different temperature range, from 40 to 140$^{circ}C$, and the droplet sizes of the heavy oils at different temperature and pump pressure were measured. And, combustion characteristics were investigated under the different conditions : two different heavy oil and three different oil temperature. At temperature of 70, 100, 130$^{circ}C$ the kinetic viscosity of heavy oil A and B are 7.9, 5.7, 4.3 and 30.4, 13.7, 7.9cSt, respectively. The greatest and smallest viscosity were 7,455 cSt at C oil on 27$^{circ}C$ and 4.26cSt at A oil on 140$^{circ}C$. The magnitude of viscosity difference between at 100$^{circ}C$ and 140$^{circ}C$ under 6 cSt in cases of A and B oil, but more than 30cST on C oil. Of the droplet sizes, the biggest and smallest droplet size in A oil were 98$mu extrm{m}$ at oil temperature of 130$^{circ}C$(4.3cSt), pump pressure of 1.57MPa and 72$mu extrm{m}$ at 70$^{circ}C$(7.9cSt), 2.35MPa, respectively. It appeared that as spraying pressure increased the droplet size decreased, however, no distinct differences were found in the effects of kinetic viscosity on the droplet sizes of the test range. The best combustion performance was observed when droplet size, spraying pressure and oil temperature were 73$mu extrm{m}$, 2.35MPa and 70$^{circ}C$ producing CO2 of 13.1%, CO of 13ppm and flue gas temperature of 250$^{circ}C$ in A oil combustion For B oil, it was100$^{circ}C$, 2.35MPa, 52$mu extrm{m}$, producing CO2 of 10ppm and flue gas temperature of 260$^{circ}C$. In general, it appeared that better combustion results were observed in the smaller droplets produced burner condition.