A novel localization method is proposed and demonstrated which exploits the phase profile combined with the location of the moving robot to locate target tags using only a single straight-line interrogator antenna trajectory. A system that integrates a mobile robot with an integrated RFID reader and antenna is used to obtain phase measurements of target tags when the robot is moving along its trajectory. The cross-range location of the target is obtained from the stationary point of the fitted phase curve. The down range distance is estimated by finding the integer number of wavelengths which fits the cross-range location and curve shape. The proposed method could reduce the localization error to around 12 cm which is similar to the SAR method using a straight-line trajectory, but with much lower computational complexity.