Despite being a popular topic in language testing, cognitive load has not received enough attention in vocabulary test items. The purpose of the current study was to scrutinize the cognitive load and vocabulary test items’ differences, examinees’ reaction times, and perceived difficulty. To this end, 150 students were selected using cluster/convenience-sampling, and took the Cambridge Placement Test (CPT) and Vocabulary Size Test (VST; Nation & Beglar, 2007). After uploading the vocabulary-size test’s items in PsychoPy software, there was a behavioral stage to measure students’ reaction times and correct responses. Out of these 150 high school students, a total of 60 (20 from each proficiency level of elementary/intermediate/advanced groups) were selected. In this quantitative study, all 60 students were interviewed to determine their perceived difficulty of the international VST items and their item’s difficulty-index. The data were analyzed quantitatively via simple regression and qualitatively through the examination of the students’ perceived difficulty. The results and interview findings revealed a significant connection between cognitive load/reaction time, difficulty estimate, and perceived difficulty at intermediate level. In contrast, at elementary and advanced levels, these variables could not predict the cognitive load. The findings can help to test, course, and syllabus designers by educating them on the significance of cognitive load theory so that they can base their exam designs on its premises and alleviate students' increased cognitive-workload.
