Objective：To investigate the effects of p38 mitogen-activated protein kinase on titanium（Ti） particle-induced osteoclast formation. Methods：Bone marrow cells were collected from the femur and tibiae of 4-6 week-old BALB/C mice and were divided in－to three groups：①control group；②receptor activator for nuclear factor-κB ligand（RANKL） treated group；③Ti particle and RANKL treated group. The numbers of osteoclasts were measured by the tartrate-resistant acid phosphatase（TRAP） staining. The expression and phosphorylation level of p38 protein were determined by Western blot. Then an adenovirus-mediated RNA interference targeting p38 was employed to knock down the expression of p38 and the numbers of osteoclasts were determined by TRAP staining；the ex－pression of osteoclast specific gene RANK was detected using Western blot. Results：TRAP staining showed that the numbers of os－teoclasts were significantly higher in Ti particle treated group than in control group with the same concentration of RANKL（blank group：2.0±0.8，RANKL group：62.8±5.6，Ti particle +RANKL group：93.0±8.8，F=235.193，P=0.000）（RANKL group vs. blank group，P=0.000；Ti particle+RANKL group vs. RANKL group，P=0.000）. The relative phosphorylation level of p38 was significantly improved in Ti particle and RANKL treated group compared with that in control group and RANKL treated group（blank group：0.29±0.05，RANKL group：0.64±0.05，Ti particle+RANKL group：0.87±0.05，F=106.491，P=0.000）（Ti particle+RANKL group vs. blank group and RANKL group，P1=0.000，P2=0.001）. After being treated with adenovirus-mediated p38 siRNA（Ad-sip38），the number of osteo－clasts and the protein level of RANK were remarkably decreased（TRAP positive cells：control group：99.8±13.5，Ad-RFP group：95.8±7.1，Ad-sip38 group：3.8±2.5）（control group vs. Ad-RFP group，P=0.541；Ad-sip38 group vs. control group and Ad-RFP group，P1=0.000，P2=0.000）. Conclusion：p38 MAPK plays an impor－tant role in the osteoclast formation induced by Ti particle.