Basic principle of ultrasonic flow meters is to measure the traveling time difference of ultrasonic wave between the go and back signals traveling through gas flows. The flow fields are one of the most important factors of the meter quality together with the sensors and the electronic systems. That is, the signal to the noise ratio of the sensor depends on the flow condition of the passage of ultrasonic wave. Typical ultrasonic flow meters are analyzed from the viewpoint of fluid dynamics, using hot-wire anemometry, visualization with PIV method and numerical simulation. The flow fields with low turbulence level are required for meter.