Most previous studies on disciplinary academic writing focused on the structures in research articles or linguistic realizations of each move (Lau, 2004; Hyland, 2000). Few have been conducted to address the interpersonal aspect of disciplinary discourse texts. The purpose of this study was to measure the effect of lexical bundles' (LBs) awareness on writing enhancement of high-intermediate ESP learners. To this aim, 16 male and female learners studying at Tunis Business School were selected. Their grades of previous academic years underlined the homogeneity of the participants. They were randomly divided into experimental and control groups. The control group was taught based on the conventional approaches of writing. The experimental group received a treatment of LBs awareness in the writing of a credible academic prose. The design of the study was based on four phases respectively a pre-test, a treatment, a posttest and a questionnaire. The pre-test was constituted of two short parts aiming at examining the rhetorical features of the participants' writings particularly their knowledge of LBs. Then the treatment phase which comprised three tasks in teaching LBs. After that, a post-test was administrated in an attempt to examine the effect of the treatment on the participants' output. Next, an 8-item evaluation questionnaire, including an open-ended question, was distributed to elicit information about the students' attitudes towards the techniques used to raise their awareness of LBs' use. The findings revealed that there was a significant difference between the writing scores and LBs' use among the participants in the control and experimental groups. The results of the questionnaire also demonstrated students' positive perceptions towards the usefulness of the LBs treatment tasks.