A 6-month-old, intact male Great Pyrenees (35 kg) was referred with 2 weeks continuous left forelimb lameness to the Chonbuk Animal Medical Center, College of Veterinary medicine, Chonbuk National University. The lameness became worse three days before being referred to the hospital. Upon the physical examination, the patient had pain when the left shoulder joint was palpated, and the lameness was visible in the left forelimb during ambulation on gait examination. There were no remarkable findings on radiological and neurological examination. Osteochondritis dissecans (OCD) was suspected based on medical history and gait tests. As a definitive diagnosis could not be made, exploratory arthroscopic surgery was performed to examine the inside of the shoulder joint. During the operation, mild bicipital tenosynovitis, synovitis and OCD which was located on the caudal medial area of humeral head were revealed. Arthroscopic procedures were used for the treating OCD, including the removal of the OCD flap and debriding of the subchondral defect until hemorrhaging by use of an electrical burr. The patient was discharged a day after surgery. After 2 weeks, the patient again presented at the hospital due to complications, including inflammation of the surgical lesion because of licking and seroma within the subcutaneous tissue. Antibiotics were administered and an aseptic bandage was applied. And simple surgical operations were performed for the removal of the cyst and seroma. Eleven weeks following arthroscopy, the lameness was completely resolved. Arthroscopy has the advantage of allowing gross examination inside the joint capsule. Due to this advantage, arthroscopy is one of the best advanced options for diagnosis in dogs with undiagnosed joint pain.