I. Introduction
The main aim of this paper is to retrieve similar design patent images based on image content accurately and rapidly for users such as patent agents and design patent inventors. These users need to check whether a patent is innovative or not. Design patent images which present the main point of the design patent are the most important part of this patent (e.g. Fig. 1). Several web sites provide patent retrieval service now in China [4], [5]. But almost all of these retrieval systems just provide text based patent retrieval. The text-based retrieval service can not meet the user's retrieval demand for similar design patent images. As a result, the search of design patents based on image content is becoming more and more important to patent attorney and inventors of design patents.