Abstract: For the purposes of gaining an insight into the characteristics of transient flow, the experiments of upliquid transient flow are conducted on the large multiphase flow loop and the transient responses are presented. Following the a ccretion of liquid flow rate, pressure waves resulting from the fluctuation of t he fluid density propagate downstream quickly and pressure of various points und ergoes disturbances. After some time, voidage waves arrive various points, as a result, the pressure build up quickly. The results demonstrate that the transmis sion speed of pressure increase approximates the voidage wave traveling at a velocity equal to slug translation velocity in case of slug flow, liquid flow velocity under stratified flow and mixture velocity under plug flow. The evolution of the pressure drop varying with the flow pattern and the liquid flow rate is different with that of the pressure in most cases. The time for the pressure drop to reach the steady value is shorter than the pressure. In the transients, the gas holdup descends slowly and tends to steady state asymptotically with small mag nitude of void fraction decrease.