Twelve allenic aromatic ethers, some of them are natural products isolated from the mangrove fungus Xylaria sp. 2508 in the South China Sea, were synthesized. Their antitumor activities against KB and KBv200 cells were determined. All these compounds demonstrated cytotoxic potential, ranging from weak to strong activity. The analysis of structure-activity relationships suggested that the introduction of allenic moiety could generate or enhance cytotoxicity of these phenol compounds.