Biodegradable plastics based on soy dreg (SD) were prepared with glycerol as a plasticizer via compression molding at 110-160degreesC. The effects of the glycerol content and molding temperature on the processability and mechanical properties were investigated. For enhanced water resistivity of the SD plastics, thermoplastic benzyl konjac glucomannan (B-KGM) films were covered on SD sheets in a lamination compression process, and SD/B-KGM (SB) composite sheets were obtained. The structure and properties of the SB sheets were studied with Fourier transform infrared spectroscopy, dynamic mechanical thermal analysis (DMTA), scanning electron microscopy, and tensile testing. The results indicated that the tensile strength of the SB sheets was enhanced to 13.2 MPa in the dry state and to 11.4 MPa in the wet state. The water resistivity of the SB sheets with B-KGM covers increased from 0.03 to 0.86 with an increase in the B-KGM thickness from 0 to 35 mum. The multipeaks of a tan delta relaxation that appeared in the DMTA spectra for SB series sheets were due to multicomponents such as soy protein, cellulose, polysaccharide, and B-KGM. The tan delta transition temperature and the decomposition temperature of cellulose in SB sheets were higher than those in SD sheets without B-KGM covers. This work provided a simple way of improving the compression-molding processability and water resistivity of SD plastics for practical applications. (C) 2003 Wiley Periodicals, Inc.