The new monomer, alpha-ethoxy-exo-3,6-epoxy-1,2,3,6-tetrahydrophathaloyl-5-fluorouracil (EETFU), was synthesized from 5-fluorouracil (5-FU) and alpha-ethoxy-exo-3,6-epoxy-1,2,3,6-tetrahydrophthaloyl chloride. Its homopolymer and copolymers with acrylic acid (AA) and vinyl acetate (VAc) were synthesized by photopolymerizations using 2,2- dimethoxy-2-phenylaceto-phenone. The synthesized samples were characterized by FT-IR, H-1-NMR and C-I3-NMR spectroscopes, elemental analysis, and gel permeation chromatography. The EETFU contents in poly(EETFU-co-AA) and poly(EETFU-co-VAc) were 40 and 37 mol %, respectively. The number average molecular weights were in range from 8,400 to 10,300. The in vitro cytotoxicities of synthesized samples were evaluated against mouse mammary carcinoma (FM3A), mouse leukemia (P388), and human histiocytic lymphoma (U937) as cancer cell lines and mouse liver cells (AC2F) as a normal cell line. The range of IC50 values obtained from the in vitro test for synthesized samples were 0.03-0.16 mu g/mL against cancer cell lines. The in vitro cytotoxicities of polymers were beter than 5-FU. The in vivo antitumor activities of synthesized monomer and polymers were also investigated by mice bearing the sarcoma 180 tumor cells. The in vivo antitumor activities of the synthesized monomer and polymers were greater than those of 5-FU at corresponding dosage concentrations.