Abstract: In order to find out the mechanism of difficulty in flotation of low-rank and oxidized coal,experimental investigation is conducted with typical low-rank and oxidized coal samples. The work made covers: size,density,infrared and XPS analysis; calculation of surface free energy of coal sample using Washburn dynamic method; calculation of interaction energy of coal-water,coal-agent and coal-watercollector by using interfacial polar interaction theory; measurement of interaction force between moleculars of paraffin/stearic acid and surface of difficult-to-float coal particle by using atomic force microscopy（ AFM） colloidal probe technique; and verification of thermodynamic calculation result using micro-nano testing technique. The exprimental result shows that: the presence of polar groups on surface of low-rak and oxidized coal particles is the key factor causing the difficulty of flotation of such coals; there exists an attractive force between the polar groups in collector and the surface of a low-rank and oxidized coal particle; and intensification of the interaction between reagent molecules and particle surface is one of the effective approaches for improvement of flotability of low-rank and oxidized coal.