Four quinazolinones, new naturally occurring 1 and three known derivatives 2–4, along with previously synthesized 4-phenylbut-3-enamide (5) and three known compounds, 2-acetamido benzamide, 1Hindole-3-carbaldehyde, and 2-phenylacetamide, were isolated from an actinomycete, Streptomyces sp. BCC 21795. The structures were elucidated based on the analysis of NMR spectroscopic and mass spectrometric data. Compound 1 exhibited strong cytotoxic activity to Vero cells with IC50 3.30 mg/mL.