The bioactive molecules from microalgae have important properties such as antioxidant, anticancer, antiviral, antibacterial, antihypertensive, skin regenerative, neuroprotective, sunscreen, and, immunostimulatory effects. Bioactive molecules derived from microalgae is getting intense attention from pharmaceuticals, cosmetics and nutraceuticals industries because of these properties and numerous researches have been done to investigate the role of these bioactive molecules that can enlighten microalgal biotechnology to result in new nature derived pharmaceutical formulations. In this study we investigate antioxidant activities of crude extracts of G. Galdieria sulphuraria, Ettlia carotinosa, Neochloris texensis, Chlorella minutissima, Stichococcus bacillaris, Schizochytrium limacinum, Crypthecodinium cohnii, and Chlorella vulgaris with determining radical scavenging activity (RSA) by DPPH (2,2-diphenyl-1-picrylhydrazyl hydrate radical) method and total phenolic content by Folin-Ciocalteu method. Selected extracts according to their antioxidant activities cytotoxicity was evaluated after exposure to human hepatocellular liver carcinoma cells (HepG2) for 48 h. Antioxidant activities of these species ranged from 89 to 95% RSA (radical scavenging activity) while their phenolic contents were also very high, varied from 41 to 312 mg GAE (gallic acid equivalents)/mg extract. MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) assay also showed microalgae has a potential to be used as a novel therapeutic compound.