Abstract: A polyester film having excellent heat resistance and insulation property is produced by the inventive process which comprises: melt-extruding a polyester resin into a sheet, extending the sheet to shape it into a film and treating the film thermally to heat-set, wherein the polyester resin has an intrinsic viscosity of 0.7 to 0.75 dl/g and other portions which satisfy formula (1) and the thermal treatment is carried out so that formula (2) is satisfied:[COOH]/[IV]<50 (1)1.0<S/D<1.2 (2)wherein:[COOH] represents the content (meq/10.sup.6) of terminal carboxyl groups in the polyester resin;[IV] represents the intrinsic viscosity (dl/g) of the polyester resin;S represents the X-ray crystal size (.ANG.) of the heat-set polyester film; andD represents the degree of crystallinity (%) of the heat-set polyester film.

Abstract: A biaxially oriented polyester film with excellent levelling property is produced by the inventive process which comprises: (a) melt-extruding and extending biaxially a polyester resin to shape it into a film; (b) treating the film thermally under a condition that formulae (1), (2) and (3) are satisfied; and (c) relaxing the film at a temperature range of Tm-60.degree. C. to Tm-140.degree. C. during the step (b):Tm-60.degree. C.<T1<Tm-10.degree. C. (1)50<T1-T2<100 (2)5<(T1-T2)/t<25 (3)wherein:Tm represents the melting point of the film (.degree.C.),T1 represents the initial temperature for the thermal treatment of the film (.degree.C.),T2 represents the terminal temperature for the thermal treatment of the film (.degree.C.), andt represents time for the thermal treatment (sec.).