License Plate Recognition (LPR) system is an important part of Intelligent Traffic System (ITS), which is very valuable in practical applications such as public safety, traffic management and military department. The license plate image edge detection is a hotspot to the process of license plate location and recognition. The edge detection affects the follow-up quality of the location and identification. This paper describes the various edge detection algorithms and operators. It gives the general steps of edge detection and analyses the existing difficulties. Through the simulation of the edge image on the plate, it analyzes the feasibility of the algorithm and summarizes the advantages and disadvantages of five detection operator which provides a theoretical basis for the next license plate location and identification.