To promote the heat-sealing properties of soy protein isolate (SPI) films applied in the packaging field, we mixed a synthetic polymer of poly(vinyl alcohol) (PVA) with SPI to fabricate blend films by a solution-casting method in this study. To clarify the relationship between the heat-sealing properties and the heat-sealing temperature, strength, melting process, crystalline structure, and microstructure, variations of the heat-sealing parts of the films were evaluated by means of differential scanning calorimetry, tensile testing, scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy, respectively. The test results showed that both the PVA and glycerol contents greatly affected the melting behavior and heat of fusion of the SPI/PVA blends; these blend films had a higher melting temperature than the pure SPI films. The peel strength and tensile strength tests indicated that the long molecular chain of PVA had a main function of enhancing the mechanical properties above the melting temperature. With increasing heat-sealing temperature, all of the mechanical properties were affected by the microstructure of the interface between the laminated films including the chain entanglement, crystallization, and recrystallization. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115:1901-1911, 2010