The biodegradable composition comprises at least poly(beta-hydroxybutyric acid) or a biodegradable copolymer thereof and a stereospecifically regular block copolymer having the structure units of formula (I): ##STR1## where R1 is a hydrocarbon group having 1 to 14 carbon atoms, optionally including double bonds, oxygen atoms or both of them; and m and n are respectively an integer ranging from 300 to 5,000, or having the structure units of formula (II): ##STR2## where R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are either a hydrogen atom or a methyl group; and m and n are respectively an integer ranging from 300 to 5,000.