Organic/inorganic biocomposite materials for biodegradable fixation medical devices require osteoconductivity, biocompatibility, and adequate mechanical properties with biodegradation behavior. The objective of this study was to investigate the effect of Si ions substituted in β-tricalcium phosphate (β-TCP) on the mechanical properties of organic/inorganic biocomposites. Biodegradable composite materials were prepared with polylactic acid (PLA) as the matrix and nano Si-substituted β-TCP as the osteoconductive filler by solvent mixing and conventional molding. The nanostructured Si-substituted β-TCP powders were synthesized by co-precipitation, controlling the quantity of Si ions. The amount of nanostructured Si-substituted β-TCP powders in composites was varied in the 0-40 wt% range and the material properties were compared with those of pure β-TCP/PLA composites. The influence of Si ions on the mechanical properties of the composites was evaluated by tensile and flexural tests.