It is very important to measure and diagnose the state of knife and strip cutting face in real time in the side trimming process due to improvement of quality of strip trimming face. In this study, a diagnosis system for knife and strip cutting face was developed. Two CCD cameras facing each other are mounted at the delivery side (the entry of the bur masher roll) of the side trimmer. This system offers the operator the diagnosis result of state of knife and strip cutting face. With the help of the developed system, the knife breakage has been detected 100% and bad quality of trimming face is significantly decreased. Using CCD cameras, an on-line diagnosis system which can determine knife breakage and bad quality of trimming face in real time is developed in trimming. A diagnosis algorithm determining the state of the strip cutting face is also developed by calculating characteristic values and image processing based on the images of the strip cutting face obtained from CCD camera.