The ultrasonic signal propagation in two-layer gas flow was studied. The intensity degradation of the signal was observed while the signal was propagating through the air-hydrogen-air two-layer gas flowing system. The concentration of flowing hydrogen (H-2) gas was measured using ultrasound from the exterior of the pipe, and it was calculated that the intensity degradation of signal did not simply depend on the H-2 concentration, however, the intensity varied every second. Schlieren photography was taken to visualize the motion of H-2 gas after injecting into the flowing air of 2 m/s. It was observed that high concentration H-2 gas was flowing in the middle of the airflow without quick diffusion into the air. A two-dimensional air-H-2-air gas flow model was considered where 100% H-2 was flowing in the middle of the airflow, and the gas layers were separated by two fluctuated interfaces. According to the calculation using this model, only limited conditions of the signals can reach to the receiver due to the refraction at the fluctuating air-H-2-air gas interfaces while propagating. It was found that the receiver could hardly detect the signals; hence, the intensity of the signal looked degraded. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.