The experiments of the effect of particle size on flameless combustion characteristics and NOx emissions of Shenmu semi-coke were launched on a bench-scale coal preheating combustion test rig. The results revealed that the difference of fuel particle size greatly influenced the yield of each component in high-temperature coal gas and the reactive activity of coal char, thus subsequently affecting the final combustion process and NOx emissions. The specific surface area and pore volume of Shenmu semi-coke were both significantly increased after preheated, leading to a better reactivity and the improved combustion performance, especially when d(50) (the mean particle size) = 163.84 mu m, where the temperature of the down-fired combustor reached the highest. The different particle size of Shenmu semi-coke resulted in different preheating temperature, which further affected the content of each component in the high-temperature coal gas. During the self-preheating process, most volatile matter and fuel bound N were separated out and the released N was mainly converted to N-2 and NH3, besides the conversion rate of each component (C, H, N and V) was the highest when d(50) = 100.17 mu m. Additionally, all experimental conditions achieved flameless combustion, with a transparent combustion area, but no flame front visible to the naked eye, and the highest combustion efficiency was 98.23% when d(50) = 163.84 mu m with the brightest flame image. It was discovered in this experimental that there was a critical value (d(50) = 198.54 mu m) of Shenmu semi-coke particles. When the particle size was lower than or higher than the critical value, NOx generated by combustion would decrease. The lowest NOx emission was 89.46 mg/m3 (@6% O-2) when d(50) was 35.34 mu m.