The isotactic polypropylene glass was prepared by quenching the molten film in dry-ice ethanol solution(below--75℃)．The specific volume of the quenched film was 1.135 cm3／g at--25℃．The relaxation and crystallization process of isotactic polypropylene glass was fol-lowed by density measurement and IR spectroscopy．Below--40℃，the thermal expen-sion coefficient of amorphous isotactic-polypropylene Was found to be 2.4X10-4／℃ and increased to 4.0 x 10-4／℃ at--40 to--20℃．Over --20℃，volume contraction took place noticeably，indicating the presence of crystallization and relaxation of the frozen maolecules．Below--23℃，there Was no change of the absorbance ratio (A998cm-1／A974cm-1) of the IR spectra of isotactic·polypropylene glass．But when the quenched film was heated to above --20℃，this ratio and the crystallization rate increased gradually．Because of crystallization．the“true” Tg of amorphours isotactie polypropylene could not be determined experimentally by conventional methods．The“true” Tg is probably just above -20℃．