Choosing academic words to use in writing an essay is one of the big problems for EFL learners today. This study investigated this issue to understand the challenges faced by EFL learners. The purpose of this study is to clarify the difficulties in using academic words, word usage, and factors affecting the choice of academic words to use in essay writing. This research included 53 students majoring in English Studies, course 44, of a university in the Mekong Delta of Vietnam. Through 53 questionnaires and 5 interview samples, the study has collected data for analysis. As a finding, this study revealed three main problems for EFL learners. Firstly, the difficulty of an academic vocabulary has affected learners' word memorization and application of academic words in essay writing. Secondly, the way to choose effective academic words to use is to create a rich vocabulary for yourself, use it regularly and improve your self-study ability. Finally, the vocabulary level of the learners and the topic are two factors that seriously affect the choice of academic words to use and the quality of the essay. In short, this study has discovered remarkable issues about the difficulties of students in the process of choosing academic words in essay writing.