Abstract: Foam stability is an important parameter affecting the flotation efficiency. To investigate the effect of diesel on foam stability,the foaming ability and foam stability of mixed solution of different diesel concentrations with 20 × 10-6 methyl isobutyl carbinol ( MIBC) solution were explored by foam scanning analyzer ( FOAMSCAN). The final status of liquid film between foams was analyzed by dynamic liquid film apparatus,and the effect of diesel on foam sta- bility was further clarified. Moreover,the effect of diesel on foam characteristics and flotation performance were dis- cussed through fine coal flotation tests and gas-liquid-solid three-phase foam stability tests. The foam stability tests of gas-liquid two-phase system showed that the foaming ability and foam stability gradually decreased with the increase of diesel concentration. The results of liquid film tests showed that the final liquid film changed from the equilibrium state to the rupture state,and became more and more unstable with the increase of diesel concentration,which eventually made the bubble more likely to coalesce or even rupture. This conclusion was consistent with the results of gas-liquid two-phase foam stability tests. In addition,the flotation results showed that the maximum froth height and half-life in- creased with the increase of diesel concentration when the concentration was lower,and the flotation clean coal yield increased as well,which was mainly due to the improvement of hydrophobicity on the coal surface and the effect of sta- bilizing foam of fine coal. However,the maximum froth height and half-life gradually decreased,and the flotation clean coal yield also decreased when the concentration exceeded a certain concentration. This is because,on the one hand, diesel invaded into the interfoam film and the film thinned driven by van der Waals force and other forces until the for- mation of classic oil droplet bridging phenomenon,eventually leading to the bubbles rupture. On the other hand,the diesel oil droplets competitively adsorbed frother molecules,so that the concentration of frother at the gas-liquid inter- face decreased,making foams less stable. Therefore,diesel has a certain defoaming effect on foam stability in flotation.