Numerical analysis has been carried out to investigate the combustion and NO formation and distribution characteristics using a beta probability density function (PDF) method in a three-dimensional municipal waste incinerator by considering the turbulent reacting flow with thermal radiation. For numerical calculation, the multiblock method is adopted to simulate a complex geometry. The program developed has been validated with a commercial program. Considering not only thermal NO but also prompt NO, parametric studies for excess air ratio, secondary air injection, and radiation have been undertaken. By increasing the primary air supply, higher excess air ratio lowers the temperature inside the incinerator so that the amount of NO formation is reduced. When the secondary air injection is considered, more secondary air injection does not always reduce NO formation because of another factor, the equivalence ratio. Additionally, in order to simulate the real situation in an incinerator, nongray gas radiation should be introduced, since including the radiation effect results in lower NO formation because of radiative heat loss.