After the occurrence of natural disasters, objective and accurate assessment of road damage is the key to emergency rescue. However, in the traditional road damage assessment, the assessment factor and the form of assessment result are single. It can only give an overview of damaged roads, but can't further provide their traffic capacities, which are essential for the disaster relief department. A new approach of road damage assessment is presented in this paper. The damage levels are graded based on the magnitude of actual traffic capacities of post-disaster roads, and the detailed damage information is output at three levels - damage block level, single-road level and region level. The proposed method is applied in the damage assessment of roads in Wenchuan County, China, which was hit by an 8.0-magnitude earthquake. The experimental result shows the validity of this method.