The decrease in bone mass caused by wear debris-induced osteolysis could have been compensated through osteoblasts secreting enough new bone matrix. However, the normal osteoblastic population depends on the regular differentiation of their progenitor cells, the bone marrow mesenchymal stem cells (BMSCs). It is not possible to predict whether wear particles will affect the BMSCs’ viability, and subsequently their differentiation. Furthermore, little is known about the extent to which the sizes of the wear particles loading can impact the viability the most. This study has, therefore, concentrated on the potential mechanism for rat BMSCs’ (rBMSCs) viability influenced by different-sized titanium particle (Ti) loading in vitro.rBMSCs were harvested and loaded with circular Ti particles having three different mean diameters, 0.9, 2.7 and 6.9 .m respectively. The results showed that different-sized titanium particles all inhibited rBMSCs’ proliferation and induced rBMSCs’ apoptosis response , but this influence varied with the size of the Ti particles, their concentration and the duration of loading. The smallest Ti particles (0.9.m) exhibited the earliest and largest suppression on the proliferation and the most powerful induction on the apoptotic response of rBMSCs. qRT-PCR analysis demonstrated that those apoptotic effects were association with the abnormal accentuation of inducible nitric oxide synthase(iNOS) activity. The size of titanium particles generated through wear of a prosthetic device and the osteoblastic progenitor BMSCs may be both important considerations in the development of superior implant technology.