Studies have shown that algae and seaweed have cytotoxic activity. This study was aimed to determine the cytotoxic activity of Spirulina platensis and Ulva compressa Linn. extracts against cancer cell lines. The cytotoxic activity of the extract was carried out using the MTT ((3-(4,5-dimetylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) method. Results showed that the ethanol extract and methanol extract of Spirulina platensis have no cytotoxic effect against HeLa, WiDr, MCF7 and T47D cells. Water extract of Spirulina platensis had no cytotoxic activity on T47D and Vero cells. Water extracts of Spirulina platensis increase MCF7 cell growth. Phycocyanin powder also stimulates MCF7 cell growth. Ethanol extract of Ulva compressa Linn. exhibited potentially cytotoxic activity against MCF7 and moderate cytotoxic against WiDR cells with IC50 values are 31.86 μg/ mL and 104.93 μg/mL, respectively. It can be concluded that extract of Spirulina platensis has no potential to be developed for cancer therapy. Ulva compressa Linn has the potential to be developed as an anti-cancer. Further research for study the mechanism of anticancer of Ulva compressa Linn on MCF7 was needed.