Condensation of 2-amino-5-aryl-[1,3,4] oxadiazoles (I) with aromatic aldehyde in ethanol afforded 2-arylidenoamino-5-aryl [1,3,4] oxadiazoles (II) in excellent yields. Heating under reflux of (II) with ammonium thiocyanate in dioxane resulted in 6,7-dihydro-2,7-diaryl-[1,3,4]oxadiazolo[3,2-a][1,3,5]triazine-5-thiones (III) in moderate yields. Treatment of the compounds (III) with N-(hydroxyalkyl)-phthalimide in concentrated H2SO4 yielded the target compounds named as 2-[(2,7-diaryl-5-thioxo-5H-[1,3,4]oxadiazolo [3,2-a][1,3,5]triazine-6(7H) -yl)alkyl] isoindolin-1,3-diones (IV) in the yields ranging from 57 to 68%. The compounds (IV) were screened for their antimicrobial activity in vitro.