In this paper, effect of surfactants on the hydrophobicity of low rank coal particles by sliding time measurements was investigated. X-ray photoelectron spectrometer (XPS) analysis indicated that the surfaces of low rank coal particle consisted of many hydrophilic functional groups. It was observed that the sliding time of low rank coal particles in 2-ethyl hexanol solution was the shortest than that of low rank coal particles in dodecyl amine hydrochloride (DAH) and sodium dodecyle sulfate (SDS) solutions. Moreover, the maximum touching angle (theta(T)) of low rank coal particles observed in 10(-2) mol/L of 2-ethyl hexanol solution was about 80 degrees, which was the greatest than that of low rank coal particles, about 39 degrees in 10(-3) mol/L of DAH and 35 degrees in 10(-4) mol/L of SDS solutions, respectively. Therefore, the sliding time results indicated that the hydrophobicity of low rank coal was enhanced by 2-ethyl hexanol and DAH and reduced by SDS. However, it was noted that the hydrophobicity of low rank coal was depressed when the DAH concentration increased to 10(-2) mol/L. The flotation performances were consistent with the results of sliding time measurements. Therefore, effect of surfactants on the hydrophobicity of low rank coal particles can be characterized by sliding time measurements.