Abstract: A novel distal location method of interlocking intramedullary nails based on gravity center of light spot is presented. The components of the measurement system are described. To measure and position distal lockholes for interlocking intramedullary nails, an image transmission fiber bundle is used to send illuminating beam, as well as receive and deliver the distal image spot of interlocking intramedullary nails to the CCD target surface. A real-time system based on VC++ was exploited to get the gravity center of the light spot. During image processing, a modified threshold transform is used to avoid the defect caused by the binary transform. A circle is drawn directly around the light spot to get the gravity center. It is easy to get the actual position of distal lockholes for interlocking intramedullary nails by computing the change of gravity center of the spot before and after deformation. In addition, a lot of measurement experiments were performed. The quantitative analysis of the result shows that the alignment accuracy is up to 0.1mm. It meets the requirement of the measurement system.