The research of marine anti-tumor drugs develops rapidly, and marine algae are kinds of unique potentially pharmaceutic materials and some of them can be cultivated. It is very important for exploiting and utilizing the resource of marine algae before studying marine drugs. This paper reviews the works on screening of anti-tumor substances in marine algae and the compounds with antitumor activities. Different methods have been applied to evaluate the anti-neoplastic activities of marine algal extracts. Antitumor activities are determined by inhibiting KB human carcinoma cells, P388 lymphocytic leukemia or Ehrlich ascites tumor. They are also evaluated in a series of mechanism based anticancer screens, including protein kinase C (PKC), protein tyrosine kinase (PTK) and inosine monophosphate dehydrogenase (IMPDH). About 70 compounds with anti-tumor activities from marine algae have been reviewed in this paper. Many of them are important, such as Halomon, which isolated from marine red alga Portieria hornemanni. More than 10 halomon-related anti-tumor monoterpenes have been identified and are useful in clinical experiment. Malynganide D is metabolites of Lyngbya majuscula, and it produces a highly cytotoxic profile against KB cells. Amphidinolid J is a cytotoxic macrolide from the marine dinoflagellate Amphiolonium sp. Other cytotoxic substances, such as okadaic acid, lyngbyatoxin A, debromoaplysiatoxin, hormothamnione, hormothamnin A, 15(28)-anhydrothysiferyl diacetate, 15-anhydrothysiferyl diacetate, magireol-A, magireol-B, magireol-C, caulilide, dexylapachol, dilopholide and caulerpenyne are also introduced.