In attempt to find new pharmacological active molecules, we synthesized 6-iodo-4H-3,1-benzoxazin-4-one and allowed it to react with some nitrogen nucleophiles namely; hydroxylamine hydrochloride, hydrazine hydrate, fomamide, aliphatic amine, aromatic amines, aralkyl amine, different amino acids, heteryl amines, ethanolamine and sodium azide to afford annelated quinazolinone derivatives and other related systems. The synthesized compounds were characterized with the help of spectroscopic techniques including IR, 1H-NMR and Mass spectra. Also their antimicrobial activities were screened against different strains of bacteria and fungi. Keywords: 6-Iodo-4H-3,1-benzoxazin-4-one; quinazolinone derivatives; nitrogen nucleophiles; antimicrobial activity .