The context clue strategy to derive the meaning of unfamiliar words and its effectiveness have been generally accepted, and the strategy is widely taught to ESL/EFL students. This study aimed to (1) compare the abilities in making use of context clues to derive the meaning of vocabulary among four groups of English major undergraduate students (Years 1-4); (2) compare the abilities to identify the context clues used to derive the meaning of unfamiliar words among the four student groups; (3) investigate the correlation between ability and context clues used to derive the meaning of unfamiliar words; and (4) investigate students’ opinions towards using the context clue strategy to derive the meaning of unfamiliar words. A 40-item vocabulary test and a set of questionnaires were administered to English major undergraduate students in the Faculty of Liberal Arts at a university in Thailand in the second semester of the academic year 2020. Then, 39 responses were randomly selected from each student batch to be the study sample. Three students from each student group were randomly selected to be interviewed to gather more in-depth information. The findings revealed that the ability to use context clues to derive the meaning of unfamiliar words among the four groups of English major undergraduate students was significantly different. Conversely, there was no significant difference in their ability to correctly identify the context clue types they used to infer word definitions. Also, there was a significant correlation between both abilities. Moreover, the students showed positive opinions towards the use of context clues to infer the meaning of vocabulary and provided some interesting ideas regarding the teaching of context clues for ESL/EFL teachers and English major program committees.