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-SP 160 (esterified by palm oil) to light yellow color for L-SP 80 (esterified by lauric acid) and showed high carbon content, ranging from 399.3 to 537.1 g kg ร1 . 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-SP 160 and L-SP were 3.23 and 2.95 to 5.18 mmol g ร1 , respectively. In addition, L-SP 80 exhibited the most appro-priate thermal softening behavior by heating. The cumulative decomposition of P-SP 160 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-SP 80 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-SP 160 and L-SP 80 remarkably influenced the decomposition rate of both molded P-SP 160 and L-SP 80 . V