In order to evaluate the mechanical properties of poly(lactate-co-3-hydroxybutyrate) [P(LA-co-3HB)] and its correlation with the LA fraction, P(LA-co-3HB)s with a variety of LA fractions were prepared using recombinant Escherichia coli expressing the LA-polymerizing enzyme and monomer supplying enzymes. The LA-overproducing mutant E. coli JW0885 with a pflA gene disruption was used for the LA-enriched polymer production. The LA fraction was also varied by jar-fermentor based fine-regulation of the anaerobic status of the culture conditions, resulting in LA fractions ranging from 4 to 47mol%. In contrary to the opaque P(3HB) film, the copolymer films attained semitransparency depending on the LA fraction. Young's modulus values of the P(LA-co-3HB)s (from 148 to 905MPa) were lower than those of poly(lactic acid) (PLA) (1020MPa) and P(3HB) (1079MPa). In addition, the value of elongation at break of the copolymer with 29mol% LA reached 150%. In conclusion, P(LA-co-3HB)s were found to be a comparatively pliable and flexible material, differing from both of the rigid homopolymers.