Copper oxide nanoparticles (CuONPs) were synthesized by a green method using an aqueous extract of Cystoseira myrica as a capping and reducing agent. A UV-visible spectrophotometer was used to characterize the produced nanoparticles. The aqueous extract of C. myrica at pH 5 and 100°C after 150 minutes was shown to be the optimal condition for the synthesis of copper oxide nanoparticles. The Structure and characters of synthesized nanoparticles were confirmed using TEM, DLS, XRD, and FTIR. The efficiency of different concentrations of copper oxide nanoparticles (25, 50, 75, 100) µg/ml synthesized from aqueous extract of C. myrica exhibit high antitumor activity against some cancer lines such as HepG2 and MCF-7. The percent of cell viability decreases as the concentration of nanoparticles rises. Copper oxide nanoparticles at a concentration of 100 µg/ml were found to have the lowest percentage of cell viability in HepG2 (18.7%), while MCF-7 was (19.27%).