an apparatus is disclosed for the fabrication of a biaxially stretched flat film of thermoplastic material , particularly of polypropylene , with improved properties for a process in which the material is first partially pre - stretched in a longitudinal direction and , thereafter , simultaneously biaxially stretched , the apparatus comprising stretching rollers for effecting partial longitudinal stretching of the film , the rollers being arranged upstream of an assembly for effecting simultaneous biaxial stretching of the film . the speed of the stretching rollers is infinitely variable so as to influence the total degree of film - stretching only by changing the speed of the rollers . the rollers used for longitudinal pre - stretching are arranged at the inlet section of the biaxial stretching assembly . a system is provided to maintain the fiml exactly at the temperature required by the film characteristics for stretching , and biaxial stretching is done such tht any given point on the film in the region the film is being biaxially streched undergoes simultaneous biaxial stretching both in the longitudinal and transverse direction of the film .