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This paper presents a novel device-free motion tracking (DFMT) system using an RF sensor network with very few nodes. The system includes a newly designed measurement model and corresponding tracking algorithm. Based on the uniform theory of diffraction (UTD), we prove that the received signal strength (RSS) measurements can be separated into two parts, the long-term one and the short-term one, reflecting the shadowing and scattering effects of the target. Then a model considering both the two effects is proposed to obtain positions of target using RSS measurements, which expands the sensing range of each wireless link. Particle filtering algorithm is applied to perform tracking. Experiment results illustrates that the system works well with very few nodes in different settings.