A process for producing a thermally treated biaxially oriented polypropylene film, the process comprising the steps of: (A) providing a polypropylene composition comprising (A1) from 90 to 99.99 wt.%, based on the total weight of the polypropylene composition, of a homopolymer of propylene having a content of isotactic pentad fraction of from 93 to 98 % determined by NMR spectroscopy and a melt flow rate MFR2 of from 0.4 to 10 g/10 min determined according to ISO 1133, and (A2) from 0.0000001 to 1 wt.% of a polymeric α-nucleating agent based on the total weight of the polypropylene composition, and/or (A3) from 0.0000001 to 0.0002 wt.% of a β-nucleating agent based on the total weight of the polypropylene composition, (B) extruding the polypropylene composition to a film, (C) orienting the film simultaneously or sequentially to obtain a biaxially oriented polypropylene film, (D) thermally treating the biaxially oriented polypropylene film by (D1) heating the biaxially oriented polypropylene film to an annealing temperature T ann , wherein T ann is between 80 and 150 °C, (D2) annealing the biaxially oriented polypropylene film at the annealing temperature T ann for a annealing time t ann to obtain the thermally treated biaxially oriented polypropylene film, wherein t ann is between 10 and 140 min, (D3) cooling the thermally treated biaxially oriented polypropylene film, (E) recovering the thermally treated biaxially oriented polypropylene film. (A) providing a polypropylene composition comprising (A1) from 90 to 99.99 wt.%, based on the total weight of the polypropylene composition, of a homopolymer of propylene having a content of isotactic pentad fraction of from 93 to 98 % determined by NMR spectroscopy and a melt flow rate MFR2 of from 0.4 to 10 g/10 min determined according to ISO 1133, and (A2) from 0.0000001 to 1 wt.% of a polymeric α-nucleating agent based on the total weight of the polypropylene composition, and/or (A3) from 0.0000001 to 0.0002 wt.% of a β-nucleating agent based on the total weight of the polypropylene composition, (A1) from 90 to 99.99 wt.%, based on the total weight of the polypropylene composition, of a homopolymer of propylene having a content of isotactic pentad fraction of from 93 to 98 % determined by NMR spectroscopy and a melt flow rate MFR2 of from 0.4 to 10 g/10 min determined according to ISO 1133, and (A2) from 0.0000001 to 1 wt.% of a polymeric α-nucleating agent based on the total weight of the polypropylene composition, and/or (A3) from 0.0000001 to 0.0002 wt.% of a β-nucleating agent based on the total weight of the polypropylene composition, (B) extruding the polypropylene composition to a film, (C) orienting the film simultaneously or sequentially to obtain a biaxially oriented polypropylene film, (D) thermally treating the biaxially oriented polypropylene film by (D1) heating the biaxially oriented polypropylene film to an annealing temperature T ann , wherein T ann is between 80 and 150 °C, (D2) annealing the biaxially oriented polypropylene film at the annealing temperature T ann for a annealing time t ann to obtain the thermally treated biaxially oriented polypropylene film, wherein t ann is between 10 and 140 min, (D3) cooling the thermally treated biaxially oriented polypropylene film, (D1) heating the biaxially oriented polypropylene film to an annealing temperature T ann , wherein T ann is between 80 and 150 °C, (D2) annealing the biaxially oriented polypropylene film at the annealing temperature T ann for a annealing time t ann to obtain the thermally treated biaxially oriented polypropylene film, wherein t ann is between 10 and 140 min, (D3) cooling the thermally treated biaxially oriented polypropylene film, (E) recovering the thermally treated biaxially oriented polypropylene film. <EOS>