There is increasing evidence that gas/liquid two phase flow behaviour in large pipes (with diameter >100 mm) differs from those of smaller pipes. It has therefore become imperative that flow correlations are exclusively derived for large pipes so as to facilitate more accurate prediction of heat transfer and pressure drop in pipes and boilers encountered in the nuclear/process/oil and gas industries. This study focuses on the pipe diameter effect on vertical annular flow, a regime of two-phase flow characterised by liquid flowing along the pipe periphery and the gas and/or mist phase flowing in the pipe core. As such there exists friction at the interface between the core and liquid film on the wall. This interfacial friction is particularly important as it critically influences the behaviour and structure of the flow. This study is based on new large diameter pipe interfacial friction factor data which was obtained from pressure drop and liquid film thickness measurements made in Cranfield University's Serpent Rig (a 101.6 mm internal diameter flow loop) with the fluids co-currently flowing upwards through a 4 m long test section. Ranges of air and water superficial velocities taken were 1.42-28.87 m/s and 0.1-1.0 m/s respectively. Significant discrepancies were found between the published correlations derived using small pipe data and our experimentally determined interfacial friction. We have therefore made attempts to develop a new correlation using a dimensionless liquid film thickness and gas Reynolds number function that well fits the large pipe interfacial friction factor data.