Effect of molecular weight on conformation, helix structure (H structure) and trans planar structure (T structure), of Poly(ethylene oxide) (PEO) has been investigated in detail by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimeter. Two main diffraction peaks at about 2 theta = 19A degrees and 23A degrees are discovered, and XRD patterns reveal that the unit cell of crystalline PEO belongs to the monoclinic lattice. The crystallinity decreases from 93.82 to 59.62 %, and the deviation of crystalline temperature of PEO-0.5 is larger than those of the other three under four reheated cycles. From FTIR results, a red shift about 11 cm(-1) is observed in the stretching vibration of -C-O-C- with increasing molecular weight, suggesting the presence of chain-chain interactions to restrict the stretching vibration of -C-O-C- in main chains. Meanwhile, the bending region of -C-C-O- at about 533 cm(-1) sensitive to tension shifts to lower wavenumber, and a new peak at about 510 cm(-1) emerges with increasing molecular weight, which is the indicator of internal tension/strain and orientation. Furthermore, the peak intensity ratios of H structure decrease with increasing molecular weight. In contrast, T structure increases dramatically. Consequently, with respect to molecular weight, the possible interactions, entanglements and tie molecules, of PEO molecular chains to explain the difference between H and T structure is proposed, which is in agreement with the experimental observations quite well.