To explore the inhibitory effect of hydroxyapatite (HA) particles with different sizes on malignant melanoma A375 cells in vitro, we synthesized 4 short rod-like HA particles using TIPS. Their mean diameters were 998.0 nm (HA1), 511.0 nm (HA2), 244.0 nm (HA3), and 71.6 nm (HA4), respectively. Malignant melanoma A375 cells were co-cultured with HA particles in vitro. Results showed that HA particles smaller than 511.0 nm in mean diameter could always inhibit proliferation of A375 cells, and nanometer-HA particles (HA4) had the strongest inhibitory effect on A375 cell proliferation and the strongest inducing effect on apoptosis. HA particles were distributed in plasma of A375 cells. The ultrastructure changes of A375 cells were found most significant in nanometer-HA particles (HA4) group. We conclude that particle size is a very important influencing factor on anti-tumor effects of HA and that nanometer-HA particle has the strongest inhibitory effect on tumor cell proliferation.