Carbon microspheres were prepared using cellulose as a precursor by hydrothermal carbonization and the composites were prepared using carbon microspheres as the filler and poly(lactic acid) as the polymeric matrix. The morphologies and structures of carbon microspheres were characterized by scanning electron microscopy and Fourier-transform infrared spectrometry. The crystallinity, tensile properties and thermal properties of the composites were tested. The results show that the tensile properties and thermal properties of the carbon microsphere/poly(lactic acid) composites were improved by the addition of CMSs.