Abstract: The sisal fiber (SF) reinforced polypropylene composites were manufactured by injection molding. The effects of fiber content，fiber length distribution，matrix types and compatibilizer types on the impact strength of the composites were studied. The distribution of individual fiber tensile strength was analyzed. The tensile strength of fibers at the critical fiber length was estimated by the modified Weibull distribution model. The impact strength of the composites was predicted. The results show that the impact strength increases with the fiber content. The matrix types have important influence on the impact strength. Owing to forming a rigid interfacial layer between the matrix and fiber，the MAPP compatibilizer has a negative influence on the impact strength，while the PP-g-GMA compatibilizer has a positive influence on the impact strength owing to forming a soft interfacial layer. The impact strength of the composite with PP-g-GMA is 21.7% higher than with MAPP at the same compatibilizer content. When the softer interfacial layer is introduced through the reactions of the natural fiber treated by KH550 silane with PP-g-GMA compatibilizer，the impact strength is 50.7% higher than that with MAPP. The predicted and the experimental values are reasonably well approximated considering the fiber orientation factor to the impact strength model.