Understanding ash-deposition mechanism and factors decreasing heat flux through a heat-transfer surface is useful for operating pulverized coal-fired boilers efficiently and reliably. We studied ash-deposition process through a monitoring test using a portable device, and confirmed that it generally consists of three main stages: formation of a powdery ash layer, particle impact and sticking, and deposit growth. We also studied an effect of ash deposition on heat-flux decrease for three types of coals through up to about 100-hour combustion tests in a large-scale test furnace. Powdery ash layers, formed during the tests for two of the coals, decreased the heat-flux dramatically. Our analysis of the ash samples showed that for one of the coals, a large amount of Na and K vapor condensed on the surface of the layers, and for the other coal, the layers contained a high percentage of fine ash particles under 10μm.