Polymer composites consisting of poly(butylene succinate) (PBS) as matrix and corn starch (CS) or cellulose filler (CF) with a small amount of furfural (FR) from biomass were prepared by a hot-pressing method at 120 degrees C from a powder mixture. Mechanical properties of the composites' films were investigated using tensile test methods. The strain of these films was found to be developed by adding CS or CF and further improved by adding FR. The strain at break PBS with 15 wt % FR was improved by more than 16 times of that of PBS without FR. The biomass carbon ratios of polymer composites with oil-based PBS (major component) and with biobased CS, CF, and FR were evaluated by C-14 concentration ratio measured by accelerator mass spectrometry (AMS) based on ASTM D6866. PBS with 40 wt % CS or CF had a biomass carbon ratio of 31 and 36%, respectively. PBS with 15 wt % FR had a biomass carbon ratio of 3.4%. This deviation was confirmed by FR content calculated from the peak area of the H-1-NMR spectrum of PBS with FR which was 3.4%, almost the same as the biomass carbon ratio measured by AMS. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3410-3417, 2009