The purposes of this study are to investigate how fracture roughness affects solute transport and is to find a better transport mechanism causing a long breakthrough tailing. To achieve the purposes, the lattice Boltzmann method was used for simulating the solute transport in rough fractures. Rougher fracture yields a large velocity gradient across the aperture, especially at high average fluid velocity (Re>20). Consequently, solute transports fast in the middle of the aperture, and a significant fraction of solute are delayed and transport slowly along the fracture wall, which can be described as the “film” transport. This “film” transport results in high solute dispersion, and was found to be a better explanation for a long tailing in the breakthrough curve, rather than the most common explanation for a long tailing, the diffusive exchange of solutes between mobile and immobile fluids. As long as fluid velocity is kept small, discrepancy between tracer and hydraulic apertures may not be significant, regardless of fracture roughness.