Abstract: To improve the prediction accuracy of the oil recovery by waterflooding,the changes in wettability and relative permeability of reservoir rocks and their effects on two-phase flow had been researched.Wettability experimental measurements and applied mathematics were conducted to characterize the correlation between the wettability of reservoir rocks and water saturation during waterflooding.The governing equations of two-phase flow were derived to fit wettability changes in oil reservoirs.And the prediction method of oil recovery with wettability variation in waterflooding of oil fields was presented.Wettability experimental results showed that the relation between the logarithm of wetting index and water saturation of sandstone cores was approximately linear.Relative permeability experimental results showed that water saturation for the equal values of water and oil relative permeabilities increases from 0.41 to 0.53 in the cores from low water-cut to high water-cut reservoirs,which also indicated the rise in water wettability.Simulation results of one dimensional model showed that the predicted oil recoveries for oil and water wetting reservoirs were 52.7% and 73.3%,respectively.The wettability variation trend and mathematical model presented in this paper lay theoretic foundation for accurate prediction for oil recovery by waterflooding.