The effects of ultrasonic irradiation on conformation and crystalline structure of isotactic polypropylene (iPP) and beta-isotactic polypropylene (beta-iPP) were studied by means of Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The results demonstrate that ultrasonic irradiation decreases the helical conformation order and changes the crystalline structures: for iPP, T-m and T-onset decrease, T-m - T-onset becomes larger; for beta-iPP, the beta(c), (1-lambda)(beta) and k(beta) increase, the intensities of alpha:(040) plane and alpha(130) plane in WAXD profiles decrease evidently. For both iPP and beta-iPP, the crystallinity decreases, d-spacing increases, the crystallite size L decreases and ultrasound shows a selective effect on the growth of beta-crystal. Furthermore, DSC and WAXD were employed to observe the effect of ultrasound on the "melting memory effect" of beta-iPP. The results indicate that ultrasonic irradiation destructs the existing "locally ordered structure" in beta-iPP melt, as a result greatly inhibits the beta alpha-recrystallization of beta-iPP samples during heating. (c) 2009 Elsevier Ltd. All rights reserved.