The purpose of this study is to clarify the fundamental features of NOx formation in oxy-coal combustion. The pure effects of various factors related to oxygen concentration, flame temperature and flame pattern on NOx formation are investigated by experimental and theoretical analysis, resulting in the following conclusions: (1) In oxy-coal combustion (CO2/O2 atmosphere), the emission of fuel-NOx increases with O2 concentration. (2) The flame pattern changes from a detached flame to an attached flame with the increase in flame temperature. (3) During oxy-coal combustion, the gasification reaction between CO2 and carbon on the surface of char particles occurs more vigorously than during combustion in air. (4) The single particle model for estimating NOx emission is applicable to the detached flame pattern. This implies that in the case of the detached flame the pulverized coal burns with a structure of premixed flame.