We report a case of cerebral actinomycosis in a 69-year-old immunocompetent woman. The patient showed a progressive worsened mental status for one week. MRI examination showed an increased size of multiple enhancing nodular lesions associated with mild perilesional edema. We performed an open biopsy for the right frontal enhancing lesion. The intraoperative finding showed a yellowish friable lesion that was not demarcated with normal tissue. Pathologically, an actinomycotic lesion with sulfur granules and inflammatory cells was diagnosed. We report an unusual case of diffuse involvement of cerebral actinomycosis. The presence of the uncapsulated friable lesion that consisted mainly of foamy macrophages and lymphocytes could explain the unusual radiological features.