Current paper proposes an effective technique to improve mechanical properties of bamboo fiber reinforced PLA (Poly Lactic Acid) by fibrillation on the surface of bamboo fibers (BFs). To improve the interfacial properties between bamboo fibers and matrix, the surface of alkali treated bamboo pulp was fibrillated by milling machine. The bamboo pulps were milled under 3 different conditions (Fibrillated Alkali Bamboo fiber : FAB). Degree of fibrillation (D.F.) was defined as water retention of the bamboo fiber. Four types of bamboo fiber were prepared in this study ; the D.F was 0 (pulp), 3.5, 4.3 and 4.8. The FAB/PLA composites were injection molded after fibrillated bamboo fibers were mixed into PLA. The effect of fibrillation on mechanical properties of BF/PLA composite was investigated.The viscosity of melted FAB/PLA was increased and the rate of crystallization decreased when the bamboo fiber was processed with large number of (D.F.). The bending strength and interfacial shear strength of FAB/PLA composites was significantly increased in comparison to that of the original bamboo pulp/PLA composite when the D.F. was selected to 3.5, compared to that of un-fibrillated AB/PLA composites. Long pull-out of the fiber with smooth surface was observed on the fracture surfaces of un-fibrillated AB/PLA composites. However, the bending strength of FAB/PLA composites was decreased when the D.F. was 4.3 and 4.8. The discussion in the paper showed that the decrease in strength of fibrillated bamboo fiber and change of fiber orientation should degrade the bending strength of FAB/PLA composite when the fiber was excessively fibrillated. This paper mentioned the optimum condition was there to enhance interfacial properties between bamboo fiber and PLA matrix.