Isolated droplet burning was conducted in a hot-air chamber for blended oils formed from mixing a heavy oil residual (HOR) and a diesel light oil (LO) at different fractions to investigate the burning characteristics of diversified heavy fuel oils (HFOs). Coke behaviors were analyzed based on measures of coke particle size, structure, burnout time, and droplet temperature history diagram during burning. The HOR amount in the droplet was found to be determinative for the coke particle sizes at different chamber temperatures for a given oil, and to the coke burnout times for different oils at a specified chamber temperature. From the same amount of HOR the pure-HOR oil tended to give a larger coke particle compared to the particle sizes of other blend oils with LOs higher than 10 wt %. The coke particle size, however, varied little with the chamber temperature when fuel was specified. The coke burnout time was definitely longer at higher chamber temperatures for equisize coke particles, but the times for different oils at each specified chamber temperature were correlative with the HOR amount in the droplet. The coke particle from pure-HOR oil might be more hollowed out, although its shell appeared to be thinner and less penetrated with holes. A few temperature inflections, which defined several characteristic temperatures of burning, were identified on the droplet temperature history diagram. This further clarified a necessary temperature, which is about 1150 K, for setting fire to the coke particles from HFOs, where the firing refers to the coke smoldering with luminescence or glowing. At rather higher chamber temperatures the coke residues formed during volatile burning should proceed into coke smoldering (oxidation) with a zero delay time after flame extinction. The article analyzed also the characteristic temperatures at flame extinction and ignition, and the peak temperature during coke smoldering.