A new phase-based localization method for RFID tags is proposed. The position of a target tag is determined relative to a minimum of 3 reference tags of know locations using phase measurements of the target tag and reference tags along 2 straight trajectories. Experiments show a 2D mean localization error of around 12 cm. Compared to other methods, few reference tags are required, and the only trajectory information required is the distance along the track where phase readings are recorded.