The use of natural fiber as a basic material in various industrial fields has done one of them in the biomedical field. Research that addresses natural fiber has been many, but research on behavior towards fiber before alkalization is still relatively small. The purpose of this study was to look for fiber materials and additives with high strength for biomedical applications. In this study sisal fiber composites and Polymethyl methacrylate composites were made as matrices. Sisal fiber is given by washing using distilled water, detergent and boiling, then sisal fiber is soaked in an alkaline solution (6% NaOH) for 4 hours. The composition of sisal fibers using the fiber volume fraction of the calendar is 20:80. Sisal / PMMA composite fabrication is done layer by layer on the mold with PMMA at first, then mixed with sisal fiber, carried out in succession up to 3 layers to get a value approximately equal to the testing standard. Composites are pressed using cold press by generating 125 kg / cm2 for 30 minutes. Sisal / PMMA composite bending test is done with reference to ASTM D790-03. The results of single sisal fiber tensile test by washing on fiber before alkalization showed that in sisal fiber with aquadesh leaching had the highest value with an average of 342, 23 MPa, while the lowest in leaching using an average of 178.991 Mpa and on boiled surfaces had average policy of 225.31 MPa. Bending test results show that the highest liquefaction strength with maximum bending stress is 32,914 MPa and maximum strain 0,180 while fiber with an average elastic modulus maximum 0,506 GPa. The results of the morphological structure analysis of composite using optical showed that the fiber distribution was not evenly distributed with the matrix and found that the remaining PMMA was not evenly stirred .