Water cut is an important parameter in reservoir engineering. For example, water cut data may be used for the estimation of reserves and ultimate oil recovery as well as reservoir surveillance and management. However it is difficult to predict water cut in water flooding reservoirs. Simulation technique could be used to forecast the water production, but it is time consuming, expensive, and often require information that is not readily available. Fractional flow analysis can easily be conducted using only a spreadsheet. However, most of the traditional models represent water cut as a function of cumulative oil production instead of production time. Note that cumulative oil production itself needs to be estimated or predicted. To this end, we derived a new model that correlates water cut and production time in oil reservoirs. It is easier to predict water production and estimate the performance as a function of production time when using this new model than when using traditional ones. The proposed model has been tested using production data from many oil wells and several reservoirs with a wide permeability range. The results have demonstrated that the proposed new model works satisfactorily in most of the cases studied.