Hyroxyapatite (HA) and its nanocomposites reinforced with 0.5, 1, 1.5, and 2 wt% graphite nanosheets (GNs) and multi-walled carbon nanotubes (MWNTs) are fabricated by means of spark plasma sintering (SPS) process. The effects of MWNT and GN additions on the morphology, mechanical behavior, cell adhesion, and biocompatibility of HA were studied. Three-point-bending test shows that the bending strength of MWNT/HA nanocomposites increases with increasing MWNT content. However, the bending strength of GN/HA nanocomposites initially increases by adding 0.5 wt% GN, and then decreases markedly as the filler content increases. Cell culture and viability test results demonstrate that the GNs with diameters of several micrometers retard osteoblast cell adhesion and proliferation on the GN/HA nanocomposite. In contrast, the addition of 2 wt% MWNT to HA is beneficial to promote osteoblast adhesion and proliferation, thereby enhancing the biocompatibility of MWNT/HA nanocomposite.