Abstract: In order to investigate the effect of particle size on flotation of coal,particles’ scale effect is exposited with geometric characteristics,interface chemistry and flotation behavior of particles. Changes of specific surface area,total pore volume,average pore size and wetting heat with particle size were investigate dusing BET specific surface instru- ment and micro calorimeter,respectively. Young-Laplace equation was used to analyze the capillary effect of collector adsorption. Effect of particle size and wettability on floatation probability was calculated by experience theory. Also,the flotation test of ultra-clean coal with different sizes was conducted. With the decrease of size fraction,specific surface area and total pore volume increase several times. At the same time,the pore of Ⅱ type increases,the average pore size becomes larger,and the adsorption of collector on the account of capillarity is enhanced. Wetting heat of fine coal frac- tions by deionized water reduces rapidly. Floatability of particles would be the largest in a proper size range. In moder- ate turbulence intensity flotation environment,keeping the hydrophobic particles in size range of -200+45 μm or hy- drophilic particles in size range of -10 μm or +45 μm will enhance the fine coal flotation. From the flotation results of ultra-clean coal and flotation middlings with different fineness,it is concluded that with the decrease of particle size, the coal recovery reduces,water recovery increases,and flotation efficiency becomes lower under the same flotation reagent.