Blends of soy protein isolate with 10, 20, 30, 40, and 50% polycaprolactone (SPI/PCL) alone or with addition of 0.5, 1.0, 2.0 and 5.0% methylene diphenyl diisocyanate (MDI) were prepared by using an intensive mixer at 160 degreesC for 10 min, and then milled through a 1 mm sieve. The blends and SPI were compression-molded at 150 degreesC for 5 min, cooled to 30 degreesC, and removed from the mold. Thermal and mechanical properties and water absorption of the blends were studied by a differential scanning calorimeter (DSC), a dynamical mechanic analyzer (DMA), an Instron, a water absorption test, and a scanning electron microscope (SEM). The glass transition temperature (T-g) of SPI was found to decrease with increasing PCL contents in the blends containing 2 wt% MDI. For 50/50 (SPI/PCL) blends, the T-g of SPI decreased with increases in MDI content. Both DSC and DMA results showed that compatibility between SPI and PCL was improved with the addition of MDI. Mechanical properties of the 50/50 (SPI/PCL) blends increased with increasing MDI concentrations. Strength of the SPI/PCL blends with 2 wt% MDI decreased, and elongation of the blends increased with increasing PCL concentration. Water resistance of the blends was improved significantly by increasing PCL concentration and by incorporating MDI.