In recent years,as China's car ownership increasing,vehicle body size measurement has become an important part of the car running safety testing.Traditional manual measurement operates with low accuracy and efficiency.Taking advantage of the image information on cars' surface and combining with the image edge detection,connected component labeling,image registration and fusion and edge localization,a visual measurement algorithm of vehicle length is presented now.Experiment results of the vehicle length measurement show that the proposed algorithm can achieve a relative accuracy of 1%,and its time consumption is less than 3 second.Cars' body size measurement can be achieved with a high accuracy and little time consumption,which offers as an effective supplement for comprehensive performance testing of vehicles and intelligent transport system.