This paper reports a combined experimental and numerical investigation on the impact of particle size on the MILD combustion characteristics of pulverised brown coal. In particular, the work investigates the volatiles release and reactions from the coal particles with a vitiated co-flow and its impact on the formation and emission of CO, CO2, and NOx. The experimental vertical furnace is 1200mm long with an internal cross-section of 260×260mm2. High volatile brown coal from the Latrobe Valley, Victoria, with particle sizes in the range of 53–125μm and 250–355μm is injected into the furnace using CO2 as a carrier gas through an insulated water-cooled central jet. The complementary numerical results are in good agreement with the experimental measurements. It is found that, for both cases, a stable MILD combustion is established with a similar large recirculation vortex around the centre of the furnace. Devolatilisation starts earlier for the smaller particles' case and is completed at the end of the recirculation vortex, while for the larger particles' case the devolatilisation happened post the recirculation vortex. The difference is related to the particle dispersion within the jet and differences in Stokes number. The particle flow path and difference in residence time had influence on char burn-out and emission levels. The formation/destruction of NO is measured to be subtly varied by the combination of the physical and chemical nature of the MILD combustion characteristics related to both particle sizes. The measured NO emission of the larger particle case is 15% higher than that of smaller particle case. The model calculated about 95.5% of total NO is produced via the fuel-NO route. A strong NO-reburning mechanism is found for both cases, where ≈55% of total NO is reduced for the small particle case and ≈39% of total NO is reduced for the large particle case. •Stable MILD combustion of pulverised brown coal is achieved.•A semi-uniform temperature distribution is measured inside the furnace.•The CO and NO emissions in the exhaust are higher for the small particles case than the large particles case.•The numerical model predicts 95.5% of total NO is produced via the fuel-NO route.