Finding more effective methods to treat cancer remains a challenge, and development of new therapeutic agents for cancer treatment is essential for continued progress against the disease. This study was conducted to evaluate the in vitro antiproliferative activity of methanolic extracts of Chlorella vulgaris and Nostoc muscorum against Human cervical Cancer cells (HeLa), liver hepatocellular cells (HepG-2), Human colorectal adenocarcinoma cells (HT-29) and Ehrlich Ascites Carcinoma Cells (EACC) in order to determine the bioacitivities of microalgae. Morphological changes of cancer cells treated with the selected extract were observed by phase contrast microscopy. The cells were incubated with increasing concentrations of microalgal extracts for 24, 48, and 72 h, and cell viability was determined by the Sulforhodamine B (SRB) assay. After treatment of cancerous cells with microalgal extract for 48 h, 55% of the cells were viable, 38% were in early apoptosis, 5% were in late apoptosis, and 1% exhibited necrosis. Further studies are needed to discover the mechanisms underlying the antiproliferative activity of microalgal extracts against cancer cell lines and to identify specific bioactive compounds that are responsible for the observed activities