Substantial amount of research has been done in recent decades for the development of nanoparticle systems to selectively deliver drugs to cancer cells for concurrently enhancing and reducing anti-cancer and off-target effects, respectively. pH-sensitive carbonate apatite (CA) was originally developed for efficient and targeted delivery of DNA, siRNA and proteins to various cancer cell lines. Recently, the CA particles were employed to deliver anti-cancer drugs, cyclophosphamide, doxorubicin and methotrexate to cancer cells. Here, we report on the fabrication and characterization of gemcitabine- loaded CA particles, followed by the evaluation of their roles in enhancement of cytotoxicity in two human and one murine breast cancer cell lines. HPLC was performed to measure binding efficiency of the drug to the apatite particles whereas particle size and zeta potential were evaluated to characterize drug/apatite complex. Depending on the initial doses of the drug, its bind binding affinity towards the particles varied from 3.85% to 4.45%. The particle size was found to surprisingly decrease with an increase of the initial drug concentration. In vitro chemosensitivity assay revealed that apatite/drug nanoparticle complexes presented significantly higher cytotoxicity to breast cancer cells compared to free drugs, which could be correlated with the enhanced cellular uptake of the small size drug-loaded particles through endocytosis compared to the passive diffusion of the free drug.