Abstract: A modified surface film renewal model based on a picture of the mechanism of the mass transfer between the gas and liquid phases is proposed.This model explains why the mass transfer rate at the beginning is faster,but will approach steady state after a certain period. A graphical presentation for calculations based on the methmatical model derived is suggested. Data for several mass transfer processes taken from the literature were examined and their respective surface renewal frequencies, 5, were calculated with results more practical than those from Danckwerts model.