We report herein new biocomposites prepared from thermally treated oyster shell powder (TTOSP) and modified polypropylene (PP). Untreated and heat-treated OSP are examined to determine which of them is more effective as a bactericidal agent that can be incorporated into a modified PP to prepare biocomposites. The results of TGA and SEM indicate poor compatibility between PP and biocomposites containing more than 0.5 wt% TTOSP. A quantitative analysis reveals that TTOSP can impart antibacterial activity. MTT assay tests on the biocomposites reveal that the relative growth rates of L929 fibroblast cells increase with the TTOSP content, indicating that the biocomposites exhibit no cytotoxicity. The results from this study suggest that a TTOSP content of as low as 0.5 wt% is sufficient to improve the thermostability and tensile property of PP.