Line feature is one of the most important features of palmprint image. A novel approach of principal palm-line extraction is proposed in this paper based on their characteristics of ridge edges. First, gray adjustment and median filtering are used to enhance contrast and weaken noise. Then, palm-lines are detected based on diversity and contrast. After this step, an improved Hilditch algorithm is used to do thinning, an edge tracking approach is applied to get rid of twigs and short lines, and then, the broken lines are connected. Finally, the single pixel principal palm-line image is obtained. The experiment results show that the principle palm-lines of most pictures can be detected correctly by using this algorithm referred above. Moreover, the algorithm can provide effective and accurate statistics for palmprint identification.