A taxonomic study of the genus Cyperus L. in Phu Chong Na Yoi national park, Ubon Ratchathani province was conducted from July 2010 to December 2012. Field exploration and specimen collections were made along eight routes representing five vegetation types at 200-300 m.s.l. of open rock platform, deciduous dipterocarp forest, mixed deciduous forest, dry evergreen forest and open area. Fourteen species and one variety of the Cyperus were recognized, i.e. C. castaneus Willd. (Kok prong), C. compactus Retz. (Ya bai khom), C. compressus L. (Kok dok baen), C. cyperinus (Retz.) J.V. Suringar (Ya liam), C. difformis L. (Kok khanak), C. iria L. (Ya rangka khao), C. laxus Lam. var. laxus (Ya tin ka), C. leucocephalus Retz. (Ya faek mai), C. pilosus Vahl (Kok cho dok khon), C. pulcherrimus Willd. & Kunth (Kok lek), C. rotundus L. (Ya haeo mu), C. tenuiculmis Boeck. (Ya dok daeng), C. tenuispica Steud. (Kok na) and C. trialatus (Boeck.) Kern (Ya sam liam).