Biodegradable plastics were synthesized for the effective use of sago starch-extraction residue, which has been discarded as a waste. Two types of esterified sago starch-extraction residue, P-SP and L-SP, were obtained. It had black color for P-SP160 ( esterified by palm oil) to light yellow color for L-SP80 ( esterified by lauric acid) and showed high carbon content, ranging from 399.3 to 537.1 g kg(-1). Biodegradable plastics from the residue, which had high esterification degree showed thermoplasticity and slower decomposition in Andisols in Japan and Inceptisols in Philippines. The esterification degrees of P-SP160 and L-SP were 3.23 and 2.95 to 5.18 mmol g(-1), respectively. In addition, L-SP80 exhibited the most appropriate thermal softening behavior by heating. The cumulative decomposition of P-SP160 in Andisols and Inceptisols showed 16.7 and 32.8% of total carbon during 31 day of the incubation. On the other hand, the decomposition rates of L-SP80 in Andisols and Inceptisols were less than 10% of total carbon during 31 day of the incubation. The addition of triacetin as plasticizer to P-SP160 and L-SP80 remarkably influenced the decomposition rate of both molded P-SP160 and L-SP80. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 3145-3151, 2011