The present study aimed to determine the hypoglycemic, cytotoxic effects and phytochemical constituents of Cystoseira trinodis and Microcystis aeruginosa extracts. The GC-MS and 1H NMR analysis of C. trinodis and M. aeruginosa extracts revealed the presence of fatty acids, steroids, terpenoids and vitamins which may play an antidiabetic role. The cytotoxic effects of those extracts were tested on the tumor cell lines (HepG2) and the hypoglycemic effects were investigated in alloxan-induced hyperglycemic mice. Fasting blood glucose level showed significant decreased on days 1, 3, 10, 17, 24 and 31 groups treated with C. trinodis extract, compared to diabetic groups. The cytotoxic effect of C. trinodis petroleum ether extract displayed LD50 value of 112 mg/ml, while chloroform extract displayed LD50 value of 77 mg/ml. The two fractions of Microcystis aeruginosa had no cytotoxic effect on HepG2 cells.