A series of 10a-phenylbenzo[b]indeno[1,2-e][1,4]thiazin-11(10aH)-ones (3) has been synthesized and tested for their antimicrobial activity. The antimicrobial evaluation data indicated that compounds, 3b, 3d, 3k and 3m exhibited very promising antibacterial activity and the compounds 3b and 3k exhibited notable activity, almost comparable to penicillin for Staphylococcus aureus and Bacillus subtilis respectively. The derivatives 3g and 3l exhibited high antifungal activity. Moreover, antibacterial activities were more prolific than antifungal activity. The QSAR studies indicated the importance of topological parameters, Kiers second order molecular index (κα(2)) and molecular connectivity index (χ) in describing the antibacterial activity and electronic parameters, the energy of highest occupied molecular orbital (HOMO) and the dipole moment (μ) in describing the antifungal activity.