The dependence of strength properties of solvent vapour-treated and pre-tensioned polypropylene films on solvent interaction parameter, chi, and non-specific cohesion force, D, and the amount of pre-tension force, has been studied. The five strength properties are yield stress, sigma(gamma), initial modulus, E, draw stress, sigma(d) tensile strength, sigma(b), and breaking factor, BF. The results show that for most solvent vapour treatments, the 98 mN pre-tensioned PP film exhibited larger values of the strength properties which also increased with decreasing plasticization effect (increasing interaction parameter/non-specific cohesion force). With the exception of tensile strength and breaking factor where the untreated PP films have larger values, the other strength properties (yield stress, draw stress, and initial modulus) exhibited smaller values than the treated PP films. These results are explained by the net balance of two opposing forces: crystallization by stretching orientation and solvent presence on the one hand, and plasticization, on the other.