Rimming flow of water that leads to a thin film onto the inner surface of a horizontally rotating cylinder is studied in this work. At higher rotational speeds, axial flow of uniform thin film is established inside the rotating cylinder. Film thickness measurements under different flow conditions were performed in the annular flow regime using an optical interferometric technique. Dimensional analysis was also performed to understand the parametric dependence of key parameters involved in the rimming flow of water inside a horizontal rotating cylinder and expressions to determine average film thickness and average residence time are also presented. This study will provide a basis to estimate the transport characteristics in the thin film inside the horizontal rotating cylinder. (C) 2014 American Institute of Chemical Engineers