Abstract: Composites of poly(1actic acid) (PLA), acorn starch(AS), and dimer fatty acid(DA) were prepared by melt extrusion. Keeping the mass ratio of PLA to AS at 60/40, the effect of DA content on the mechanical, hydrophobic, thermal properties and melt flowability of the composites was studied by tensile test, flexural test, melt index analysis, contact angle analysis, scanning electron microscope, dynamic thermomechanical analysis and thermogravimetric analysis. It showed that the hydrophobic property and melt flowability of the composites increased, while the tensile and flexural strength decreased with increasing DA content. DA could not only improved the toughness of PLA matrix, but also reduced the glass transition temperature and the early thermal stability of composites.