The island of Taiwan was formed at a complex convergent boundary between the Philippine Sea Plate and the Eurasian Plate. Due to geographical factors in Taiwan, bridge engineering is an important issue. The steel bridge often used in urban areas, attributed to its advantage such as lighter self-weight, easier constructability, so there are many research about its repairing and maintenance. Nowadays, corrosion on steel bridges is identified by human eye, but identifying through visual processing has already been proposed in previous studies. In past studies, they used small size image to recognize rust. In recent years, the popularity rate of smartphone has been raised, and smartphone has been equipped with high-pixel camera and good CPU. If this research can develop an application for smartphone, it can not only improve efficiency but also reduce the equipment costs for bridge engineers. This research develops a steel rust recognition system by using edge detected technology, in order to reduce image processing time and improve efficiency. By parameters testing and adjustment, the research used real rust image to test the system. Finally, research develops a smartphone application for real-time rust recognition.