The morphology of the coir fiber by alkali treatment and untreated was analyzed by scanning electron microscope (SEM). The main component of wax layer on coir fiber surface and the mechanical performance of coir fiber/line low density polyethylene (LLDPE) bio-composites were studied. The flexural and impact strength of the composites after alkali treatment for the fibers were lower obviously than that of the composites without treatment. The result of Infrared Spectrumetry-Fouorier Transform (FTIR) shows that fatty acid and waxy material on the surface of the coir fiber were disposed of by alkali solution. The fatty acid and waxy material are compatible with non-polar LLDPE matrix. The compatibility between the coir and matrix was reduced obviously with loss of them. Therefore, the alkali treatment weakens significantly the compatibility between the coir and LLDPE, and this is a main reason that the mechanical performance of the composites was markedly declined.