Background: Cancer is a disorder when abnormal body cells proliferate out of control and invade other organ sites. Objective: Our goal is to study how the marine sea grass Cymodocea rotundata (CR) suppresses breast and liver cancer growth by inducing cell cycle arrest in vitro. Materials and methods: Using high performance liquid chromatography (HPLC), the phenolic chemicals in CR were identified. The anti-proliferative effects on MCF-7, HepG-2 and normal cell of HSF were evaluated using the MTT test. Using flow cytometry, the cell cycle arrest processes were examined in both cancerous cell lines. Lastly, utilizing quantitative RT-PCR, the expression level of BCL-2, survivin, CDC-2, and CC2D1A and P53 genes was examined. Results: HepG-2 and MCF-7 cell growth was concentration-dependently inhibited by the seagrass extract, while normal cells HSF was not adversely affected. The S phase cell cycle arrest was indicated by a marked drop in the G0/G1 phase and an increase in S phase cells. A quantitative real-time RT-PCR study for CR seagrass on HepG-2 concluded that CR extract showed a significant decrease in the expression levels of the genes BCL-2, Survivin. Additionally, compared to control cells, it also showed a significant increase in the expression of the CC2D1A, CDC-2 and p53 gene. Moreover, BCL-2, survivin, and CDC-2 expression levels were markedly increased, however CC2D1A and p53 expression levels were markedly decreased in MCF-7. Conclusion: CR may prove to be a unique adjuvant in the treatment of liver and breast cancer.