Hydroxyapatite nanoparticles (nHAPs) have been recognized for potent antitumor effects in certain cancer cells, making them good candidates as drug delivery agents and tumor therapeutics with fewer than normal side effects. This study is aimed to correlate cell proliferation inhibition with the size and morphology of nHAPs in a human breast cancer cell line as well as in normal tissue cells. We present our in vitro experimental evidence that nHAPs with sizes smaller than 50 nm have high inhibitory activity against human MCF-7 breast cancer cell lines. Based on our experimental data, normal fibroblast cells (NIH 3T3) were relatively more viable upon treatment with the nanoconstructs. The present study indicates that nHAPs can be engineered as nontoxic specific inhibitors as efficient breast cancer therapeutics in humans.