This experiment was conducted to investigate thermal and mechanical properties of the biodegradable composites based on Poly (lactic acid) (PLA) incorporated with cellulose-graft-polystyrene (cellulose-g-PS) at 1, 3, 5 and 10 wt%. The modification of cellulose filler was confirmed by the Td shifting of thermogravimetric analysis (TGA) thermogram from 334 to 348 °C and characteristic peaks corresponding to the styrene benzylic ring obtained from fourier-transform infrared spectroscopy (FTIR) analysis. The slight improvement of PLA properties with addition of 1 wt% of cellulose-g-PS composite probably was due to the good interaction and compatibility between filler and the matrix of polymer.