Abstract: A process for producing a polytetrafluoroethylene porous film having excellent mechanical strength wherein pores have a substantially perfect circle and it is possible to uniformly control its pore size to a predetermined size. The porous film is obtained by compression molding polytetrafluoroethylene resin molding powders obtained by a suspension polymerization method to prepare a preform, sintering said preform at a temperature of at least 327.degree. C., forming it into a film, resintering the film at a temperature of at least 327.degree. C., quenching it at a cooling rate of at least 70.degree. C. per hour to reduce the crystallinity of polytetrafluoroethylene to 55% or below, and thereafter stretching said film at a stretch ratio of from 1.3 to 6.5 while heating it to a temperature of from 100.degree. to 320.degree. C.