The effect of particle size on flotation performance of hematite and quartz was investigated. Microflotation, X-ray photoelectron spectroscopy analysis, reagent adsorption measurements, and collision and attachment probability calculation between particle and bubble were conducted in this investigation. The results showed that the floatability of minerals with different particle size fractions was different, which was mainly related to surface bonding site, reagent adsorption, collision probability and entrainment. The quartz with different particle size had little impact on hematite recovery, but -45 μm fraction negatively affected Fe grade of concentrate both in the direct and reverse flotation of hematite. In the direct flotation, the Fe grade in froth product dropped off due to the fine quartz entrainment. While in the reverse flotation, the Fe grade in sink product dropped off as a result of difficulty in floating fine quartz particles, which was due to lower collision probability. Meanwhile, in the reverse flotation, the presence of hematite fines (-18 μm fraction) also had negative impact on hematite recovery because of fine particle entrainment.