Sisal, carbon, and poly-methyl methacrylate (PMMA) are the component materials that have been developed for the biomedical composite. However, characterization of the mechanical properties of the composites affected by some modified treatments is still opened for discussion. Sisal/poly-methyl methacrylate (PMMA) and sisal/carbon/PMMA composites with 30% fiber content and 6 mm fiber length were manufactured using a cold press molding at room temperature for about 60 min curing time. Tensile and bending properties of the composites were investigated by the influence of alkalization, the addition of maleic-anhydride-grafted polypropylene (MAPP) and hybridization of sisal and carbon fibers. The results indicated that the addition of MAPP (3, 5 and 10 wt. %) increases the tensile and flexural strengths of sisal/PMMA composites which are higher than the composites reinforced with alkali-treated and untreated sisal fibers. The addition of 5 % MAPP resulted in more effective improvement in mechanical properties compared to the effect of alkalization. However, a significant enhancement of tensile properties was shown by the hybridization effect of sisal and carbon with a ratio of 1:1 and 1:2 in sisal/carbon/PMMA composites. Scanning electron microscopy (SEM) of tensile fracture surfaces confirmed the presence of a functional relationship between the high mechanical strength of the composites with excellent adhesion between sisal fiber and PMMA by introducing 5% MAPP. Relatively homogeneous fiber dispersion in the matrix either sisal fibers or mixed sisal and carbon fibers within the PMMA matrix with sisal/carbon ratio of 1:2 have also contributed to the improvement of the mechanical strength. The use of alkali-treated sisal and HNO3-treated carbon fibers had promoted a remarkable increase in tensile strength of the sisal/carbon/PMMA hybrid composites.