PURPOSE: To improve the biodegradability, water resistance, and strengths of a plant polymer component by mixing it with water, a compatibilizer, and a modifier, molding the mixture, and degassing, drying, and thermally modifying the resulting molded item.

CONSTITUTION: 100 pts.wt. plant polymer component comprising 50-90wt.% starch and 50-10wt.% fibers is compounded with at least 10 pts.wt. water, 540 pts.wt. volatile compatibilizer having a b.p. of 75-160°C, and 0.1-20 pts.wt. modifier, heated to 80-140°C, and vigorously mixed. The resulting compsn. is molded at 100°C or lower or at 120-180°C and degassed and dried, giving a molded item having a water content of 10wt.% or lower. The item is thermally modified at 160-220°C in the state of being fastened tightly or left open.