The invention provides a kind of ultrasonic flow meter. A measuring flow path including a pair of ultrasonic transducers is located in a portion of a minor flow path formed by using dividing member to partition inside the major flow path. From the measurement of the ratio of flow rate and the area of flow path, the total flow rate of the major flow path can be calculated. The minor and the measuring flow paths are paralleled with flow direction located in the major flow. By this structure, the fluid flowing in the major flow path won't form irregular swirl and stagnation point along the dividing member. And, it is not necessary to enlarge the distance between a pair of ultrasonic transducers and the ultrasonic flow meter provides low power consumption and high accuracy.