The crystal structure, phase change property and thermal stable temperature (T-d) of polyethylene glycol octadecyl ether [HO(CH2CH2O)(n)C18H37, C18En] with various numbers of repeat units (n = 2, 10,20 and 100) as phase change materials (PCMs) were investigated using temperature variable Fourier transformed infrared spectroscopy (FTIR), wide-angle X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TG). C18En crystallizes perfectly at 0 degrees C; and the crystal structure for n >= 20 is a monoclinic system. The number of repeat units has great effect on the phase change properties of C18En. The thermal stable temperature increases rapidly with increasing the number of repeat units. They approach to that of PEG-2000 as the number of repeat units is more than 10. T-d increases rapidly with increasing the number of repeat units. C18En are a series of promising polymeric PCMs. (C) 2013 Elsevier B.V. All rights reserved.