We use the airborne laser scanning (ALS) to scan the exposed dry surface of Nan-Shin river in Taiwan with a nominal point density of 100 points/m 2. Numerous studies showed that the fractal dimension is a good measure of the roughness of the gravel surface. The fractal dimension can be estimated from the slope of the log-log plot of the variogram. Because the gravel surface exhibits the anisotropic property, we first use variogram maps of the gravel surfaces to determine the anisotropic directions. Since the gravel surfaces show the self-affinity property, the directional variograms are then used for calculating the fractal dimension in the directions of maximum and minimum continuity. This study may lead to a better understanding of the roughness of the gravel surface using ALS data for the large-scale areas.