Preparation and Characterization of Bioactive Co-Base Alloy Composite Reinforced with Nanobioactive Glass Cobalt – based alloys form on their surface a thin (1–4 nm) passive film which consists of a mix of cobalt, chromium and molybdenum oxides thus Cobalt – based alloys are bioinert and after implantation in body a fibrous texture would be formed around it. In this study bioactive composite from cobalt – based alloy and nano bioactive glass was synthesized. For this purpose metallic alloy powders were mixed with 10, 15 and 20% nano bioactive glass. Then the mixtures were pressed and sintered via two step sintering method. Temperature range at the first step was 900-1100°C and time of this step was 10 min and temperature range at the second step was 700-900°C and time range of this step was 2-14 hours. X-ray diffraction was done in order to phase analysis. Bioactivity of samples was determined via immersion in simulated body fluid after 7, 14, 21 and 28 days. Scanning electron microscopy was used to evaluate surface of samples before and after immersion. The pH variations and also Ca, P and Si ions concentrations were measured via ICP-OES method. Bonds that were formed in samples were determined by FTIR method. Density of samples was determined via immersion method. Cell culture test in order to evaluate bone cells growth and adherence of them on surface of samples and also MTT test for composites biocompatibility determination were done. Maximum density of samples including 10, 15 and 20% nano bioactive glass were 96.2±0.3, 93.9±0.3 and 86.8±0.2 percents of the theoretical density respectively. These results were achieved when first step sintering temperature was 1000°C and second step sintering time was 10 hours. After sintering no phase changes were observed. SEM study revealed agglomeration of nano bioactive glass particles with increasing amount of them in composite. In first days of immersion samples in simulated body fluid, pH increased and after then decreased. This observation was because of reactions in order to precipitation of carbonate hydroxy apatite. With increasing reinforcement amount, number and dimension of carbonate hydroxy apatite precipitations which were formed on surface of samples were increased. Calcium ion measurement revealed by increasing amount of reinforcement quantity calcium ion release increases and composite including 20% reinforcement had highest ability to absorb calcium ion among other composites. As time of immersion and also amount of nano bioactive glass increased, silicon ion release increased and caused to increase number of apatite nucleation sites. It was found that just after one day lapse of cell culture, cells bonded to surface and a suitable primary contact with cells and surface had been formed. On 7 th day of cell culture test bonds between cells were also observed that indicating