3-Chlorobenzo (b)thiophene-2-carbonyl chloride (1) was prepared when cinnamic acid was refluxed for 48hr with thionyl chloride in chlorobenzene. Compound (1) on stirring with anthranillic acid in pyridine furnished 2-(3’-chlorobenzo (b) thiophene-2’-yl)-3, 1-benzoxazin-4-one (2). Reaction of compound (2) with hydrazine hydrate in dioxane under reflux for 3hr gave 3-amino-2-(3’-chlorobenzo (b) thiophene-2’-yl) quinazol-4-one (3), Compound (2) on refluxing with phenylhydrazine in dioxane for 3hr yielded 3-phenylamino-2-(3’-chlorobenzo (b) thiophene-2’-yl) quinazol-4-one (4). Compound (2) was refluxed with substituted anilines in dioxane for 3hr to gave 3-substituted phenyl-2-(3’-chlorobenzo (b) thiophene-2`-yl) quinazol-4-ones (5), (6), (7), (8) and (9). All newly synthesized compounds were characterized by IR, 1H NMR and mass spectral studies. These compounds were evaluated for their antimicrobial activities.