The influence of strain rate on three dimensional orientation of polyethylene tere-phthalate (PET) films has been studied by means of refractometry in the case of un-iaxial and biaxial (including simultaneous and sequential) stretchings.Two birefringence values ΔXY=nX-nYand A(Xy)Z=(nX+nY)/2--nZ (where Z axis perpendicular to the film plane

X and Y are axes within the film plane) have been suggested to evaluate the orientation of film specimens. It is shown that the uniaxial stretching mainly results in the anisotropy within the film plane

while the biaxial stretching technique is conducive to improving the degree of planar orientation in the films.