Abstract: The emittance of the ash from coal combustion is a key parameter of heat transfer in the furnace and signifi- cant to design and safe operation of power plant boilers. To systematically present the current states of the coal ash emittance and provide some reference for the determination of the coal ash emittance in practical applications,the ma- jor influencing factors and prediction models were summarized. The major influencing factors of the coal ash emittance are the wavelength,temperature,surface structure and chemical composition. The coal ash is not a gray body and its spectral emittance varies with wavelength. The wavelength and surface structure are the major influencing factors of the spectral coal ash emittance. The temperature and surface structure are the major influencing factors of the total coal ash emittance. Fe is the major chemical element affecting the coal ash emittance. However,the chemical composition has a comparatively small influence on the coal ash emittance,which depends on the wavelength,temperature,surface structure. Most research in the coal ash emittance was qualitative and it is not easy to give a quantitative conclusion. The prediction models of coal ash emittance can be divided into two groups,e. g. models based on physical processes and the empirical and semi-empirical models derived from the experimental data. Models in the former group involve calcu- lation related to scattering and determination of the optical constants which can be relatively complex. Although the empirical and semi-empirical models have simple expressions,they also have many constraints and narrow applied ran- ges. There are few empirical and semi-empirical models simultaneously taking into account various influencing factors and widely used,therefore the measurement of coal ash emittance is necessary. Emittance data supplement and estab- lishment of database will be of great help to the practice application and perfection of prediction models.