The rectorite (REC), a form of layered silicate, was facilely intercalated and even exfoliated in soy protein isolate (SPI) matrix. Furthermore, the reinforced biodegradable nanocomposite sheets were produced, in which the exfoliated REC lamellae plays a key role. After solution-mixing, XRD patterns showed that the REC lamellae were intercalated and even completely exfoliated for 4 wt % REC added, but the expanded gallery gradually became narrower with increasing REC content. FT-IR also verified the molecular-level associations between SPI molecules and REC lamellae by vibration variances of hydrogen bonding. The compression-molding further promoted intercalation and exfoliation, namely the 8 wt % REC can also be almost dispersed as exfoliated lamellae, while the interlayer of RECs also further separated for nanocomposites with higher REC content. TEM images visualized the transfer from exfoliation to intercalation and the decrease of interlayer distance with increasing REC content. The thicker and longer exfoliated REC lamellae resulted in high load and easy-to-yield of material. The maximum strength of nanocomposite sheets occurred at the addition of 12 wt % REC. Thereafter, the SPI chain can move more easily because of weak interaction between free negative-charge rich domain of SPI and REC surface in gallery, which did not favor enhancing mechanical performance. (c) 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3367-3377, 2007.