Pore volumes injected (PVI) has been found to be difficult to represent the longevity and intensity of water flooding in oil reservoirs. In order to find a better parameter to characterize fluid flooding, a new concept referred as to "flushing coefficient" has been proposed, which is a function of time and position. The effects of the viscosity, relative permeability of water phase, pressure gradient, and flooding time are considered in the flushing coefficient. Water flooding experiments were conducted in sandstone rock samples with different permeabilities ranging from about 1.0 to over 1000 md at room temperatures. The experimental data were used to test the new concept.