Coal combustion boilers in Japan use various coals, which differ in their contents of volatile matter and the nitrogen compounds, causing different combustion efficiencies and NOx emissions. In these boilers, it is desicable to achieve a high efficiency with low NOx emission regardless of the coal type used. This paper aims to examine a method to estimate the NOx concentration at a furnace exit. Using a laboratory-scale furnace, combustion tests were carried out on 12 different coals, and the NOx concentrations in the exhaust gas and the distribution of combustion gas species near the burner zone were measured. A simple one-dimensional furnace model was used to derive factors affecting the NOx emission, in which the power law was applied to correlate the NOx concentration at the furnace exit with the derived factors. The derived correlation equation was then examined with data obtained for 2 more coals.