A novel series of phthalazine derivatives bearing isoindol-1,3-dione moiety were synthesized by treating ethyl{4-[4- (1,3-dioxo-1,3-dihydroisoindole-2-yl)-phenyl] phthalazin-1-yloxy}acetate (3) and {4-[4-(1,3-Dioxo-1,3- dihydroisoindol-2-yl)phenyl]phthalazin-1-yloxy} acetic acid hydrazide (7a) with various chemical reagents.The newly synthesized compounds were characterized on the basis of their spectral (1H NMR, 13C NMR, Ms, IR) analyses. In vitro, most of the synthesized derivatives were screened for their antitumor activity against MCF-7 cells using MTT assay. Compounds 16b, 18, 13, 15, 17 showed the most potent cytotoxic effect concluded from their IC50 values 50, 70, 150, 180 and 100 μg/ml respectively.