This article presents the fabrication of cellulose nanofibrous mats (CNM) reinforced soybean protein isolate (SPI) composite with high visible light transmittance. The CNM was composed of cellulose nanofibers generated from electrospinning technique. The microstructure Of the fractured surface of composite films was characterized by scanning electron microscopy (SEM). The light transmittance, mechanical properties, and swelling ratio of CNM/SPI composite were investigated in terms of CNM content ill the composite. Because of the ultrafine diameter and superhigh length-to-diameter ratio of nanofibers, large amount of cellulose nanofibers fibers distribute in the SPI matrix to form an interpenetrating network (IPN) like composite material. It was found that Strong interfacial interactions occurred at the cellulose nanofiber/SPI interfaces. The incorporation of 20 wt % cellulose nanofibers ill the SPI matrix resulted in great improvement of mechanical strength and Young's modulus by respectively 13 and 6 times more than neat SPI film. More interestingly, this composite was translucent with light transmittance of over 75%, at 700 nm. Furthermore, the swelling ratio of this IPN-like CNM/SPI composite decreased from 106 to 22% as CNM content increased from 0 to 20 wt %. (C) 2008 Wiley Periodicals, Inc.