Actinomycosis is a chronic suppurative and granulomatous bacterial infection characterized by contiguous spread, abcess formation and sinus tract formation. There are four clinical forms according to the lesional site, as 1) cervicofacial, 2) thoracic, 3) abdominal, and 4) disseminated form. Recently, we experienced a case of 54 year-old patient with left mandibular actinomycosis. The pathognomonic findings of actinomycosis is sulfur granule with multiple filaments in Gram-stain and the treatment of actinomycosis is surgical excision of mass or sinus tract with massive antibiotics(esp. Penicillin) therapy for 6 to 12 months.