Abstract: Experimental investigation was carried out to study the pressure wave speed and its attenua- tion for gas-liquid bubbly flow in vertical upward tube. In order to improve the measurement accuracy of the pressure wave, a setup was designed to adjust the frequency of the pressure force acting on the piping flow without changing the two-phase flow regime. The propagation of the two-phase flow pressure wave was measured with several pressure transmitters mounted flush to the pipe wall along the flow direction. The re- sults showed that the wave speed in bubbly flow decreased fleetly with the void fraction and decreased slowly when void fraction was more than 0.05. The attenuation had increased continuously with the void fraction. The flow velocity of the fluid mixture had not any effect on the propagation of pressure wave. Both the wave speed and attenuation coefficient had close relationship with the perturbation frequency, which increased with the disturbed frequency under low value range. A new phenomenon about critical disturbed frequency of the pressure wave dispersion in bubbly flow was validated in the present experimental investigation, which the dispersion of pressure wave would disappear under the perturbation frequency greater than a critical value. The critical frequency was about 300 Hz in the experimental test.