Purpose : To evaluate the efficacy of arthrocentesis for treatment of closed lock. Material : 42 patients were diagnosed to closed lock by physical, radiographic examriation and undergone arthrocentesis. All patients have a pain and mouth opening limitation on affected site. Method : Arthrocentesis was done under conscious sedation and local infiltration anesthesia, normal saline and some steroid was injected on upper compartment of tempormandibular joint. After pumping and lavage, manual reduction procedure of anterially displaced disc was done. All the patients wear an anterior repositioning splint just after arthrocentesis. The result of arthrocentesis was assessed by pain and difference of mouth opening. Result : Difference of mouth opening after arthrocentesis was improved to 18.85mm and pain was gradually decreased. All patients were worn stabilization type splint after mean 1.84 months. 6 patients had relapse of mouth opening limitation so done arthrocentesis again and delivered good results.