High density polyethylene/tricalcium phosphate/ultra high molecular weight polyethylene (TCP/HDPE/UHMWPE) Nanocomposite as an orthopedic biomaterial (with better properties toward TCP/HDPE composite) was obtained. To evaluate the capability of this nanocomposite as a material for bone tissue replacement, mechanical and biological assessments were performed. In this study, nanosize β‐TCP powders with average grain size of 100 nm were synthesized by chemical precipitation method and characterized by means of X‐ray diffraction (XRD), Fourier‐transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). To evaluate the mechanical properties of this biomaterial, tensile properties were obtained for the material. Results showed that by increasing the weight percentage of β‐TCP, the elastic modulus increases, elongation at yield decreases and with no significant change in tensile strength. SEM micrographs of cryogenic fracture surface of samples indicated that distribution of nano powders in matrix is homogeneous. In vitro biological evaluations on the samples were done by performing cytotoxicity (MTT assay), alkaline phosphatase enzyme, and cell attachment tests. In all of the tests, osteoblast cells were used. Results of biological tests showed that the samples are biocompatible and they have no toxicity. Also, SEM observations demonstrated that the cells can attach to surface of nanocomposite samples, which reveals osteoconductivity of the surface. POLYM. COMPOS., 31:1745–1753, 2010. © 2010 Society of Plastics Engineers.