Biodegradable plastics were produced from sweet potato pulp (SPP) and cationic starch CS) or chitosan composite (CC) by compression molding and their mechanical properties were tested. A universal testing machine, Rockwell hardness tester, and Izod impact tester were used for testing the mechanical properties (flexural strength, Rockwell hardness, and Izod strength) of the plastics, A central composite second-order design was used to study the effects of temperature, time, and moisture content on the flexural strength, Rockwell hardness, and Izod strength of SPP/CS and SPP/CC blended plastics. The flexural strength, Rockwell hardness, and Izod strength of SPP-based plastics was 101.1-305.9 kg/cm(2), R29.0-R96.7, and 0.6-3.0 kg cm cm(-2), respectively. Regression analysis predicted the optimal mechanical properties (flexural strength, Rockwell hardness, and Izod strength) to be attained with a 150-160 degreesC temperature, 15-20-min reaction time, and 20-23% moisture content. (C0 2002 John Wiley & Sons, Inc.