Actinomycosis of the greater omentum is rare. A case of histologically proven actinomycosis of the greater omentum was reported. Ultrasonography (US) showed an omental lesion with complex echogenecity. Computed tomography (CT) revealed a mass at the greater omentum with speculated margin near the epigastric region. Actinomycosis of the greater omentum, although rare, should be included in the differential diagnosis of an omental mass especially in patients with fever (or leukcytosis).