A series of N‐alkylphthalazinone were synthesized by the reaction of phthalazin‐1(2H)‐one derivatives with alkylating agents namely, propargyl, allyl bromide, epichlorohydrin, 1,3‐dichloro‐2‐propanol, 4‐bromobutylacetate, and 1‐(bromomethoxy)ethyl acetate to give the corresponding N‐alkylphthalazinone , , , , , and . Alkylation of phthalazin‐1(2H)‐thione to give a series from S‐alkylphthalazine and thioglycosides and was performed. Deprotection of compounds , , , and resulted in the formation of the corresponding products , , , and . The structure of newly synthesized compounds was assigned by IR, 1H, 13C NMR, and elemental analysis. Some of these compounds were screened for antiviral and antimicrobial activity.