A graphical method for calculating the number of non-ideal plates of a binary distillation column has been developed. The method assumes complete mixing plates of the vapor and liquid phases. The composition of the low boiling point component in the exit vapor from a plate is not in equilibrium with that in the exit liquid from the plate. Other than these, the same assumptions adopted by McCabe-Thiele's graphical calculation method for an ideal number of plates are used. Equations of lines containing the mass transfer coefficients as parameters are deduced from the mass balances of the low boiling point component and total mass, and the number of non-ideal plates are successfully calculated by repeated drawings of the equations. The method is also applicable for the calculation of the number of non-ideal plates for a counter current extraction operation under certain conditions.