Abstract: Three fractions of different density ( -1． 4 g/ cm3,1． 4 -1． 8 g/ cm3 and +1． 8 g/ cm3 ) fine coals sized-0． 074 mm were separately mixed with the coarse coal sized +0． 074 mm,and then a flotation experiment was carried out to study the effect of different density fine coals on coarse coal flotation. The interaction force between -1． 4 g/cm3,+1． 8 g/ cm3 fine particle with the low ash coarse coal was identified separately by AFM. The concentrate and tailings were observed by SEM and the mechanisms were discussed through the extended DLVO theory. The results show that the middle fraction (1. 4-1. 8 g/ cm3 ) greatly reduces the flotation recovery of the coarse coal,the fraction of -1．4 g/ cm3 also has a depression on that of coarse coal,however,the fraction of +1. 8 g/ cm3 has less effect. The AFM test confirmed that the presence of hydrophobic force between two hydrophobic particles. The greater surface hydrophobicity,the larger hydrophobic forces. The SEM shows that middle fraction fine coal obviously covers the surface of coarse coal particles.