A localization method based on omni-directional ultrasonic sensing is proposed in this paper, circumventing the detection-angle limitation of ultrasonic signals. Experiment setup includes four ultrasonic sensors located on the vertexes in a square environment serving as receivers and a mobile robot carrying an omni-directional ultrasonic device as a transmitter. Each ultrasonic sensor is integrated with a Zig-Bee module for communication. By sequential ultrasonic signal transmission between the robot and the receivers, the ultrasonic sensors can then measure the time-of-flight (TOF) while avoiding interference to calculate the distance between the receiver and transmitter ends. According to an established two-dimensional coordinate model using dual-circle derivation, the coordinate of the robot can be obtained based on the distance measurement. Experimental results have shown a satisfactory accuracy of the coordinates of the mobile robot via the proposed localization scheme.