In order to understand the bowing phenomenon in the tentering process of biaxially oriented polypropylene film, a simple mathematical model has been developed by considering the overall continuity equation and relevant momentum equations. The model has been solved analytically by employing a lubrication approximation for an isothermal, Newtonian fluid with variant densities depending upon the external deformation. Both velocity and stress profiles in the tenter process and also birefringence and extinction angle profiles can be calculated accordingly. Geometrical bowing is strongly dependent upon the degree of volume change during extensional deformation. Theoretical prediction is compared with experimental birefringence data with two types of PP resins and two different operation conditions to give us excellent agreement. It can be said that this model is applicable to other biaxially oriented film processes such as PET.