The Effect of Amount and Type of Nanobioceramic on Mechanical Properties of Co-based Alloy Composites Reinforced with Nanobioceramics Progress in osseous surgery causes to improve biomaterials. Among the modern materials used to orthopedic implants cobalt- based alloys occupy an important place. It is estimated that about 50% of artificial hip prosthesis are made of Co-Cr-Mo alloys (vitallium). Short life of current total hip replacement implants is generally due to the aseptic loosening of the implant, which occurs due to (i) mismatch of elastic modulus between bone and metallic implant materials leading to stress-shielding, (ii) poor interfacial bond between the host tissue and the implant due to bioinert surface, (iii) wear induced osteolysis and aseptic loosening in metal-on-polymer implant. In order to increase the in vivo lifetime of metal implants one can decrease its effective modulus to match that of bone and increase the interfacial bond between living cells and implant materials compositional or structural modification. Use of nano-bioceramics such as fluorapatite, hydroxyapatite and forsterite, which are biocompatible and bioactive, and composite fabrication, could be successful. In presented research the composite materials based on Co-Cr-Mo alloy with 10, 15 and 20wt% of fluorapatite, hydroxyapatite and forsterite nanopowder as reinforcement were fabricated. The effect of type and amount of nano-bioceramic reinforcement on the mechanical and bioactivity behavior of composites were evaluated. Composites were fabricated by mixing Co-base alloy matrix and bioceramic reinforcement in planetary ball mill for 1hr, cold pressing under 750 MPa and then sintering at 1100°C for 4hrs. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for characterization and evaluation of the phase composition, crystallite size of reinforcements and particle distribution of composites. Density, microhardness, compressive strength, elastic modulus and bioactivity of fabricated composites were evaluated. To assess bioactivity bahavior of composites, the immersion test was carried out in a simulated body fluid (SBF) for 1-28 days. Obtained results showed elastic modulus of composite materials based on Co-Cr-Mo alloy reinforced with 10, 15 and 20wt% of fluorapatite, hydroxyapatite and forsterite nanopowder decreased significantly. With increasing in content of the reinforcement, elastic modulus of the composites was decreased which decreases the effect of stress-shielding. Results also showed that the compressive strength of Co-base alloy composites reinforced with 10, 15 and 20 wt% fluorapatite, hydroxyapatite and forsterite were lower than cast