摘要:The present study examined the effect of the keyword method and word-list method instruction on ESP vocabulary learning across proficiency levels. Two groups of students at Islamic Azad University of South Tehran branch were selected with the total population of 120 Industrial engineering intermediate students with an average age of 21. The students of each class were divided into two different homogeneous groups, the upper-intermediate level and the lower- intermediate level, based on their scores on the Oxford Placement Test. Each class was instructed through a specified method of vocabulary learning, i.e. the keyword method and word-list method, for an equal time of four weeks. Following both treatments, a multiple-choice test was administered to each class as the post-test to find out the ESP students' vocabulary knowledge. The results of the study indicated that the upper-intermediate learners who received the keyword method instruction outperformed the upper-intermediate learners who received the word-list method. In addition, the lower-intermediate level students in the keyword method group had better performance than those in the word-list method. Moreover, regardless of the proficiency level of the students, all of the students in the keyword method group obtained higher scores than those in the word-list method group. The results of the study revealed that the keyword method instruction has superiority over the word-lists method in learning ESP vocabulary regarding the proficiency level of the students.