End mills with small diameter have found their wide application with the development of high-speed cutting. It becomes more and more important to develop effective methods to monitor and control the milling process with small end mills. In this paper, a measuring system of projection image for small end mills is introduced, and the application of the projection image to monitor the behavior of the end mill is discussed. It is found that for static state of the end mill, the measurement accuracy can be easily controlled within 1 μm. When the end mill rotates, it is not so difficult to control the accuracy within 3 μm. By using of the change in image width, the radial wear of end mill can be predicted. On the other hand, if the centre shift of the image is pre-measured, the deflection of the end mill during cutting can be predicted.