A 11-year-old gelding Thoroughbred horse was presented with a history of unilateral nasal discharge of right side. The horse was diagnosed as maxillary sinusitis based on blood examination and radiography. The horse was treated with bone flap of right maxillary sinuses. Abscess of maxillary sinus was drained and irrigated with antibiotic-saline and povidone iodine solution. After surgical procedure, nebulization and medication applied by the result of antibiotic sensitivity test. After treatment for two weeks, the patient was recovered completely.