The genus Carex L. (Cyperaceae), with at least 2000 species, represents one of the most common vascular plant groups in the world [1]. They occur in very different habitat conditions, both in wet and moist locations such as peat bogs, fens, meadows, and pasture communities, as well as their peripheries. They also exist in dry and extremely dry habitats, which include, among others, xerothermic and psammophilous grasslands [2]. Plants of the Carex genus are characterized by the production of stilbene derivatives [3]. Also they include flavonoids [4], phenolic acids [5], essential oils and saponins [6], and fatty acids [1, 7]. In this paper, we report the constituents identified from the essential oil of Carex pseudofoetida Kuk. ex Ostenf., which is one of several types of Carex that grows wild in the north-facing Alborz Mountain Range in Iran [8]. The aerial parts of the C. pseudofoetida species were collected at Kandavan, Alborz Mountain Range, Iran, in May 2007. The aerial parts of the plant after grinding were submitted to hydrodistillation. The yield of the oil was 1.5% based on dry plant weight. The essential oil analyses were performed simultaneously by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed with an HP6890 chromatograph equipped with a flame ionization detector (capillary column, HP-5MS (5% phenylmethylsiloxane) (30 m 0.25 mm, film thickness 0.25 m). Injector and detector temperature were set at 240 C and 220 C, respectively. The GC-MS unit consisted of a Hewlett-Packard computerized system comprising a 6890 gas chromatograph coupled to a 5973 mass spectrometer. Identification of essential oil components was achieved by peak matching as well as by comparisons of retention times with authentic samples [9]. The composition of the oil of C. pseudofoetida is given in Table 1. Twenty-nine components were identified in the oil of C. pseudofoetida, which represented 98% of the total composition of the oil. Cyperene (31.5%) was the dominant constituent.