A benzoquinone, named alopecuquinone, was isolated from the ethanol extract of the inflorescences of Cyperus alopecuroides. Its structure was primarily elucidated by spectroscopic analysis including 1 H, 13C NMR, APT, HMQC, 1 H–1 H COSY and CIMS. The known flavonoids, vicenin 2, orientin, diosmetin, quercetin 3,30 -dimethyl ether and its 3,40 -dimethyl ether, were also isolated and characterized. The ethanol extract of the plant material showed moderate estrogenic activity using a strain of Saccharomyces cerevisiae.