This study was designed to identify the bioactive constituents of the red alga Gracilaria dendroides and one green alga Chlorella vulgaris and Microcystis sp. (cyanobacteria) by GC /Mass and HPLC analysis and assay the cytotoxic activity of the methanolic extract from the three species against three types of human cancer cell lines (liver HEPG-2, colon and breast MCV-7). The research was performed as an in vitro study. The effect of extract on proliferation of cell lines was measured by Methyl thiazolyl tetra zolium (MTT) colorimetric method. The results showed that the compounds identified by GC-MS were 19 compounds in G. dendroides, 11 compounds in Chlorella and 12 compounds in Microcystis,. The most abundant compounds were fatty acids, methyl esters and terpenoids. HPLC analysis identified 4 compounds in Gracilaria and 6 compounds in Chlorella and Microcystis. Crude extract of Gracilaria had the strongest activity on HepG-2 cell lines with IC 50 value 15.46 μg/ml. Concerning MCF-7 cell lines, the most potent crude extract was the Chlorella vulgaris (IC 50 value of 15.53 μg/ml). On the other hand, evaluation the cytotoxic activity for the three algae species extracts against epithelial colorectal adenocarcinoma cells (CaCO-2) showed higher activity for Chlorella vulgaris with IC 50 = 14.63 μg/ml. Accordingly, we can say that Chlorella had the strongest activity on MCF-7 and (CaCO-2) cell lines while HepG-2 cell lines most affected by Gracilaria extract.