Four types of open graded friction courses (OGFC) Marshall compacted samples with different air void contents were scanned with the X-ray computed tomography (CT). The air voids were obtained through digital image processing (DIP) of two-dimensional images. The difference of the air voids distribution on the horizontal and vertical section of the OGFC sample was analyzed. The horizontal and vertical cores were obtained by drilling the Superpave gyratory compactor (SGC) sample respectively. The permeability coefficients along twelve directions on the horizontal and vertical plane of each core were tested respectively using a developed constant head permeameter and a proposed permeability anisotropy test procedure. Results show that the air void on vertical section of the OGFC sample has more uniform size,content and quantity than that on the horizontal section. OGFC mixture has more interconnected air voids in the radial direction as compared to the vertical direction. Permeability anisotropy is existed inside the OGFC mixture. The degree of permeability anisotropy on the vertical plane is more than that on the horizontal plane of OGFC mixture. On the vertical plane,the permeability in the horizontal direction has the maximum value;while the minimum permeability was found when the water flow was in the vertical direction of SGC sample.