Indonesia possesses abundant marine biological resources, making it an ideal source for medicinal materials and treatments. Among these resources is Caulerpa racemosa, a species of green algae commonly known as the sea grape, which harbors a diverse array of secondary metabolites containing bioactive compounds with cytotoxic properties against cancer cells. Cancer remains a formidable global health challenge, particularly impacting women, with factors such as uncontrolled cell division and metastasis contributing to its severity. Caulerpa racemosa is one type of green algae that can be utilized as anti-cancer. The caulerpenin content in Caulerpa racemosa shows bioactivity against human cell lines and has anticancer, antitumor, and antiproliferation properties. This study aimeds to determine the anticancer effect of methanol extract and Caulerpa racemosa nanoparticles on MCF-7 breast cancer cells. The formulation of extract nanoparticles utilized the ionic gelation technique. The results showed that the concentrations of nanoparticle preparations used were successively: 15; 30; 60; 120; 240, and 480 μg/ mL, with an incubation time for 48 hours. Additionally, tThe results showed that the methanol extract has a cytotoxic activity, with IC50 of 38.29 ± 3.2µg /mL in the active category. And Moreover, nanoparticle preparations have a cytotoxic activity against MCF-7 cancer cells, with an IC50 value of 12.35 ± 2.8 µg/ mL in the very active category. Based on these results, it appears that Caulerpa racemosa nanoparticle preparations have a higher cytotoxic activity than methanol extracts against MCF-7 breast cancer cells.