Pea starch nanocrystals (StNs) were incorporated into a soy protein isolate (SPI) matrix to produce a class of full-biodegradable nanocomposites. The StN with low loading level (2 wt%) showed a predominant reinforcing function, resulting in an enhancement in strength and Young's modulus. This was attributed to uniform dispersion of StN in the amorphous region of the SPI matrix, as well as maintaining stress of the rigid StN and transfer of stress mediated by interfacial interaction between the active StN surface and the SPI matrix. As a result, the nanocomposite containing 2 wt% StN had the maximum strength and Young's modulus in all the materials. With an increase in StN content, the number and the size of StN domains simultaneously increased due to a strong self-aggregation tendency of StN. It lowered the effective active StN surface for interaction with the SPI matrix and destroyed the ordered structure in the SPI matrix, resulting in a gradual decrease of strength and Young's modulus. The introduction of relatively hydrophilic StN did not cause an obvious decrease of water resistance for any of the nanocomposites. The water uptake behavior of all the nanocomposites similar to that of neat SPI material was attributed mainly to the strong interfacial interaction between the StN filler and the SPI matrix. POLYM. COMPOS., 2009. Published by the 2008 Society of Plastics Engineers