A modified Ruland's method is presented for the calculation of PET fiber crystallinity. By applying the method

we can get rid of the effect of imperfectivity in crystal structure on the value of crystallinity

so that the value can really represent the percent content of crystal part in samples and possesses the correct meaning. The effect of many factors on the diffraction intensity profile is considered

meanwhile a new method for resolution of amorphous halo is given

and a FORTRAN program is also written for the calculation of crystallinity by the use of microcomputer. The results are satisfactory.