This paper presents a new technique to locate a device moving along a known trajectory, when it is equipped with an UHF RFID reader. The proposed technique exploits the phase of consecutive readings of a single stationary tag positioned near the device trajectory, within the reader coverage area. To increase the length of the monitored path where the reader can be effectively localized, the localization algorithm can be easily extended to process readings acquired from multiple tags aligned along the mobile running trajectory. A preliminary measurement campaign showed that a decimeter order accuracy can be achieved by employing a few tag readings.