Natural fiber reinforced composites have been used in automotive industries. For example an automotive industry from Germany: i.e. Mercedes-Benz wich has developed and used the polymer composite material in a product of Mercedes-Benz E Class. However, research on the natural fiber reinforced composites has been carried out until now to improve the mechanical properties of the composites. In this work, modification of fiber surface by alkalization for various duration was done to verify the effect of alkalization time on the tensile strength of kenaf reinforced polypropylene (PP) composites. Kenaf fibers were alkalized using 6% NaOH for various duration of 10, 24 and 36 h. Alkalized kenaf/PP composite were fabricated with a compression molding at 165° C, 25-30 kg/cm2 and the fiber content of 20%. The composite specimen were tensile tested with a universal tensile machine (UTM). Tensile test specimen were prepared according to ASTM D368. Tensile fracture surface was characterized by scanning electron microscopy (SEM). A optical microscope was employed to characterized fiber distribution in the composite from the cross-section view. The result showed that tensile strength of a composite specimen reinforced by alkalized kenaf for 36 h (49,94 MPa) is higher than by alkalized kenaf for 10 h (46,5 MPa). Higher tensile strength of the composite is related to be a good bonding between the fiber and the matrix surface and relatively homogeneus fiber distribution