A method for distinguishing between pulmonary arteries and pulmonary veins and a method for quantifying blood vessels are disclosed. The method for distinguishing between pulmonary arteries and pulmonary veins includes: forming a set of pulmonary vessels for points corresponding to pulmonary vessels, wherein each of the points of the set of pulmonary vessels has weight information; forming a tree from the points of the set of pulmonary vessels by using the weight information; and distinguishing between the pulmonary arteries and the pulmonary veins by separating the tree into a plurality of regions. The method for quantifying blood vessels includes: extracting blood vessels as a three-dimensional set of voxels based on medical images of an organ; finding the voxels of blood vessels included in a region of interest of the organ; and quantifying length information of the blood vessels by using the found voxels.