An investigation of the characteristics of flowrate transients within slug flow was conducted in a large-scale outdoor testing facility. The test section consisted of a 378m long, 7.62cm diameter stainless steel pipe. Air and water were used as the test fluids. The response to a change of flowrate of either phase or two phases was measured using a series of pressure transducers and differential pressure transducers. An increase or decrease in gas flowrate caused a pressure overshoot above the value at new steady state or led to a pressure undershoot to form a temporary stratified flow. Pressure waves existed in the pipeline, spreading from the entrance to the exit. The magnitude of pressure overshoot in “up-gas” transient or of pressure undershoot and period of the temporary stratified flow in “Down-gas” transient are related to the change of gas flowrate and the distance away from the entrance. In contrast, the change in liquid flowrate was accommodated by smooth transitions between the corresponding st An investigation of the characteristics of flowrate transients within slug flow was conducted in a large-scale outdoor testing facility. The test section consisted of a 378m long, 7.62cm diameter stainless steel pipe. Air and water were used as the test fluids. The response a change of flowrate of either phase or two phases was measured using a series of pressure transducers and differential pressure transducers. An increase or decrease in gas flowrate caused a pressure overshoot above the value at new steady state or led to a pressure undershoot to form Pressure waves existed in the pipeline, spreading from the entrance to the exit. “The magnitude of pressure overshoot in” up-gas “transient or of pressure undershoot and period of the temporary stratified flow in” Down- gas "transient are related to the change of gas flow rate and the distance away from the entrance. In contrast, the change in liquid flow rate was accommodated by smooth transitions betwe en the corresponding st