PURPOSE: To manufacture a high strength polyester film by a method wherein a longitudinally stretched film is laterally stretched by the specified elongation percentage.

CONSTITUTION: Longitudinally stretched film having the index of double refraction nMD, which is represented by the general formula 1, of 0.110W0.140 is laterally stretched by the draw ratio of 300W460% and thermoset at 160W220°C. Next, the resultant film is laterally stretched again by the draw ratio of 110% or more and thermoset again. By adopting the above-mentioned specified draw ratios as stretching conditions, high strength polyester film is obtained without stretching again longitudinally.