Sisal fiber reinforced polypropylene composites were prepared using an internal mixer. Heat treatred sisal fibers were performed by heating the fibers in an oven at 150, 170, and 200°C under an atmospheric pressure and a presence of air. The composites prepared at a fiber content of 20 wt% were molded by an injection molding machine for mechanical tests including tensile and impact properties. Morphologies of the composites were examined using a scanning electron microscope (SEM). Viscosity at various shear rate of the composites were also investigated. Tensile strength and impact strength of heat treated sisal fiber/polypropylene composites were slightly higher than that of the untreated sisal fiber/polypropylene composite. However, no remakable effect of treatment temperatures on the mechanical and rheological properties of the composites was observed. SEM micrographs revealed that the heat treatment improved adhsion between the fiber and PP matrix.