In the Japanese electric power industry, it is desirable to reduce the cost of treatment and expand the range of use of fly ash, such as in concrete admixtures. Therefore, it is necessary to form high-quality fly ash. To obtain high-quality fly ash, it is important to clarify the factors affecting its properties. In this study, the factors affecting the density and specific surface area (Blaine value) of coal fly ash were investigated on the basis of experimental results obtained using our combustion test facility and the ash data from the boiler of an actual electric power plant. The density was affected by the ash particle size, the true density of the component materials, and the aluminum content, which is closely related to the fusibility. The specific surface area was affected by the particle size distribution and particle shape. The shape was affected by not only the ash particle size but also the unburned carbon concentration and the ash fusibility. It was also found that the specific surface area of the ash generated from our combustion test facility is higher than that of ash from the actual boiler for ash with the same particle size. This result indicates that the shape of particles is affected by their heating and formation histories in the boiler. On the basis of the above findings, correlation equations were obtained for the density and Blaine value.