English writing as a productive skill, is the most challenging skill for EFL students. Nowadays, writing cohesive texts is of great importance especially the essay writing. Consequently, this study is concerned with the use of synonymy as one of the lexical cohesive devices in EFL students' essay writing. This study aims at identifying the most frequently used synonyms in Iraqi university students' essay writing. It also aims at investigating the way in which the use of synonyms as a lexical device contributes in building-up a well-built cohesive essay. Twenty male and female/ third-year students in the English Department, College of Education for Humanities, University of Anbar-Iraq were purposively chosen to participate in writing twenty descriptive essays. A qualitative research design was used in data collection using writing task instrument. Thus, data were analyzed qualitatively using content qualitative analysis procedure. Findings revealed that the frequency of synonymy occurrence in the students' essays was more than that of near-synonymy type. It was also indicated that the students used different synonyms and near-synonyms in their essays to achieve the goal of logical cohesion though they have a little knowledge of this type, synonymy, of lexical cohesive devices. It was also revealed that they have problems in the selection of the appropriate synonyms in their writings due to the lack of sufficient vocabulary knowledge.