A new system of Automatic Vehicle Classification (AVC) has been verified by means of a reverse engineering approach using Behavior Tree (BT). This approach was inspired by accuracy problem possessed by the AVC system as well as the benefits offered by BT. Although the AVC system has been deployed, it performs several failures in classifying vehicles with certain behavior under specific environment. It was due to specific requirements that had been overlooked by conventional textual-narrative requirement expressions. A formal but simpler and more natural way of requirement expression in Behavior Tree is expected to eliminate the weakness of conventional requirement representation. Results of the case study are BT representation of the AVC system and several test case specifications using Linear-time Temporal Logic (LTL) notations.