The strength properties of solvent vapour-treated pre-tensioned polypropylene (PP) films have been investigated. The results showed that the variation of properties (yield stress, draw stress, initial modulus, tensile strength and breaking factor) were determined by the net-balance of two opposing effects: orientation crystallization from pre-tensioning and solvent presence one the hand, and plasticization by residual solvent defined by the interaction parameter, chi, or non-specific cohesion force, D, of the halo-solvents, on the other. The greater influence of crystallizaton was shown in the draw stress initial modulus and yield stress for chloroform- and carbon-tetrachloride-treated PP films. The tensile strength and breaking factor seemed invariant with solvent-vapour treatment but still showed a greater influence of stretching orientation brought about by the greater pre-tensioning.