Frontal instability occurs in every phase of reservoir simulation (water flooding, steam flooding, chemical and carbon dioxide flooding) and in multiphase fluid flow. Whenever saturation front changes with time and spatial distribution, S(x, t), the key issue is usually the resolution of the changes of fractional flow curve with saturation (df/dS). That also implies evaluation of the derivative of relative permeability curves with saturation. An analytical technique is developed to evaluate the required derivatives and the results are used to estimate the recovery of gas displacing oil with changes in mobility ratio. The proposed technique can be accurately used to develop an algorithm for fractional flow curves. The technique is very attractive as a diagnostic tool for inquiring into the necessity for extensive reservoir simulation studies and should enjoy popularity with field engineers.