(3H)‒quinazolin‒4‒ones were reported for enhanced antibacterial, antiviral, antifungal and antimalarial activities with aryl and styryl substituents at position two and three respectively. In the present work, synthesis of some newly substituted (3H)‒quinazolin‒4‒ones starting from 2‒aminobenzoic acid and acetic anhydride producing 2‒methyl‒benzoxazin‒4‒one (Step‒I) has been undertaken. Replacement of ring oxygen from benzoxazin‒4‒one (step‒II) with nitrogen atom of 4‒chloro aniline produced first series of title compounds (QIa-QIk). In addition, 2-methyl group of benzoxazin‒4‒one was extended further as ethenyl (‒CH=CH‒) linkage joining to various substituted aldehydes (Step‒III). It has produced sufficient quantity of 2‒styryl‒3‒aryl‒(3H)‒quinazolin‒4‒ones (QIa‒QIk, Scheme I). In obtaining second series compounds (QIl‒QIt), identical approach was followed with replacement of benzoxazin-4-one ring oxygen with nitrogen atom of various substituted amines (Step‒II).There is extension of methyl group of benzoxazin‒4‒one as ethenyl group (‒CH=CH‒) joining 2‒chloro benzaldehyde. It has produced sufficient quantity of 2-[2-(2-Chloro-phenyl)-vinyl]-3-aryl‒(3H)‒quinazolin‒4‒one (QIl‒QIt, Scheme II). Structures of synthesized compounds were established by elemental analysis (C, H, N, O and X=halogen), FT‒IR, 1H NMR, 13C NMR and HR‒MS. In-vitro disk diffusion assay was performed using Gram‒positive bacteria Bacillus subtilis (NCIM2711), Staphylococcus aureus (NCIM2079), Gram‒negative bacteria Kleibsella pneumonia (ATCC 4352), Pseudomonas aeruginosa (ATCC27853) and fungi Candida albicans, Aspergillus niger. Ciprofloxacin and Griseofulvin were the positive control standards for bacteria and fungi respectively. Excellent antimicrobial activity (MIC=9.07‒10.54 µM x 10-3) was exhibited by compounds as (QId), (QIf), (QIg), (QIi), (QIj), (QIk), (QIl), (QIm), (QIn), (QIq), (QIr) and (QIs). Present study of (3H)‒quinazoline‒4‒ones will serve as an important path in the development and discovery of newer antimicrobial agents.