The effect of high pressure (HP)treatment on the conformation of freeze-dried soy protein isolates (SPI)was investigated by Fourier transform infrared (FTIR) spectroscopy. Within the amide Ⅰ′ region (1 600-1 700 cm-1)of the deconvoluted FTIR curve of SPI, more than 10 bands associated with protein conformation were distinctly observed, attributed to the CO stretching vibration and to a small extent to C—N stretching vibration of the peptide bonds, respectively. The secondary structure of native SPI is estimated to be composed of 15%-16% α-helix, 39%-44% extended strands, 17.5% random coils, and 21%-27% turns. The analyses of intensity and wavenumber of the bands showed that, HP treatment at pressures of 200-400 MPa resulted in the increases in intensity and a “red-shift” (about 2 cm-1)of these bands. HP treatment at 600 MPa further increased the band intensity of the amide Ⅰ′ region. The analyses of amide Ⅱ bands showed that HP treatment led to gradual increases in intensity and absolute area of amide Ⅱ bands, in a pressure-dependent manner. Thus, it is suggested that HP treatment resulted in gradual unfolding of secondary and tertiary structure of SPI, while the structure of denatured proteins underwent a “rebuilding” process after the release of high pressure. These results confirm that the HP-induced modification of SPI is by means of the HP-induced conformational changes.