Cyperus L. (Cyperaceae) is a worldwide genus. In Vietnam, P. H. Hô described 78 species in “Cay co Vietnam” and N. K. Khôi revisited the Cyperaceae with 61 species in “Flora of Vietnam”. The present study used the specimens that were collected from disturbed areas in 23 protected areas of Vietnam during 2003 to 2015. The taxonomic treatments were carried out using the method of comparative morphology (inflorescence, flower and nutlet,...). This study has identified 40 species of genus Cyperus. A classification key for these species was established. Each species was updated for nomenclature, described the morphological characteristics in details, along with photographs, line-draw illustrations and a distribution map. Potential new species of Cyperus for Vietnam is Cyperus surinamensis Rottb.