The vortexing effect on reducing pollutant emission was verified via the combustion of three groups of sawdust with different diameters in a vortexing fluidized bed combustor (VFBC). The temperatures and pollutant emission distributions along the combustor were measured simultaneously. The combustion fraction in four zones of the VFBC were also calculated on the basis of the oxygen consumption. Additionally, the effects of operating variables, such as the in-bed stoichiometric oxygen ratio and the excess oxygen ratio, on the pollutant emissions were also investigated. The results show that the combustion fractions in the bed and freeboard zones increase with larger particle sizes. However, an inverse trend occurred in other zones. The CO emissions from the three sawdust combustions decrease with an increase in the particle size, excess oxygen ratio, and in bed stoichiometric oxygen ratio, whereas the NOx emissions exhibit an inverse trend. The vortexing effect, which is caused by secondary air injection, demonstrated its effectiveness in prolonging the residence time of sawdust in the high-temperature zone. However, the lowest value of CO emission is still much higher than the minimal requirements of the Taiwan Environmental Protection Agency regulations. Increasing the particle size or compacting the sawdust into pellets are two possible solutions for decreasing the CO emission.