Abstract: The Xray diffraction whole pattern (XDWP) data of semicrystalline poly(ethylene terephthalate) PET2 sample were collected in the symmetric reflection geometry on a diffractometer employing a copper target (Kα wavelength 0.15418 nm )(see Fig.1 ).The XDWP was fitted by combining the Rietveld method and Fourier filtering technique,Rietveld structure refinement for PET2 crystalline phase was performed,the unitcell parameters obtained are a =0.445 nm,b=0.592 nm,c =1.072 nm,α= 99.6(°),β= 116.9(°),γ= 111.9 (°),the density of the crystals is dc = 1.495 g•cm－3(see Table 1) .Meanwhile the XDWP was separated into two parts,CR2 crystalline region and AM2 noncrystalline region in PET2 (see Fig.2).The reduced radial distribution function G2(r) was calculated from AM2 and lead to three main peaks,G21(r) = 0.150 nm,G22(r) = 0.254 nm,G23(r)=0.460 nm which characterize the structure of the noncrystalline phase in PET2 (see Fig.3).The result shows that this combination of techniques is feasible.The auther suggests that the reliability of evaluation on two phases separation and structure analysis in semicrystalline polymer,besides the three criteria to estimate the parameters of the crystalline phase,also depends on other two new physical criteria.Effect of various fitting plans on the results of structure refinement and phase separation were discussed.It is shown that in order to get reliable results it is necessary to have scattering pattern of pure amorphous sample as initial values of background intensity in fitting process. 　　This work discusses how to deal with the difficulties and its possible solutions in applying Rietveld method to polymer structure analysis.

Key words: Poly(ethylene terephthalate), Crystalline phase, Noncrystalline phase, Twophase separation, Rietveld method, Fourier filtering technique, Radial distribution function (RDF)