Today vehicle theft rate is very high, thus tracking/alarming systems are being deployed with an increasingly popularity. These systems however bear some limitations such as high cost, high false-alarm rate, and easy to be disabled. This paper describes the design, implementation and evaluation of a Sensor-network-based Vehicle Anti-Theft System (SVATS) to address these limitations. In this system, the sensors in the vehicles that are parked within the same parking area first form a sensor network, then monitor and identify possible vehicle thefts by detecting unauthorized vehicle movement. When an unauthorized movement is detected, an alert will be reported to a base station in the parking area, which sends warning messages to the security office. This paper focuses on the technical issues specific to the system such as topology management, theft detection, and intra-vehicle networking.