The surface morphology plays a significant role in floatability, wettability and hydrophobicity of mineral particles. However, the existing investigations for revealing the relationship between surface roughness and wettability of mineral particles are not sufficient. Meanwhile, there are nearly no existing researches about relationships between surface roughness and coal particles due to the fact that coal is a mixture of organic and inorganic materials and hence the study is difficult to achieve. The aim of this paper was to investigate the influence of surface roughness on natural wettability of coal. Taixi ultra-low ash coal particles were used. In order to obtain coal particles having different surface roughness, coal particles were polished using sand papers with different meshes. The surface roughness of coal particles was characterized using scanning electron microscopy (SEM) and Mitutoyo SJ-210 surface roughness measurer, and the natural wettability of coal particles was characterized using contact angle measurement Due to the measurements of SEM and surface roughness were conducted at the gold-coated coal surface, both the wettabilities of natural and gold-coated coal surface were obtained in this investigation. Throughout this paper, it was found that the surface roughness had a significant effect on the natural/gold-coated wettability of Taixi ultra-low ash coal. This paper indicates that the wettability of natural/gold-coated coal surface increases with the increase of surface roughness first, decreases second, and then has a little increase (C) 2015 Elsevier B.V. All rights reserved.