The researchers investigated the usage of appropriate academic English vocabulary in research papers authored by English major students, publications in Thai national journals, and international journals. In total, 708 papers published between 2016 and 2018 were analyzed, consisting of 300 international research papers, 300 national research papers, and 108 research papers authored by English major students. The study focused on four aspects of lexical evaluation to analyze the vocabulary in these papers: lexical profiling, lexical level, lexical variation, and lexical density. The analysis yielded the following results: 1) Lexical profiling revealed that the usage of academic words in students' research papers was 8.11%, which was lower than that in national research papers (10.96%) and international research papers (12.01%). 2) In terms of lexical level, students' research papers had a medium-frequency word usage of 3.85%, lower than that of international research papers (4.17%) but higher than that of national research papers (3.32%). 3) Concerning lexical variation, the research papers authored by English major students had a rate of 24.26%, which was lower than that of national research papers (25.01%) but higher than that of international research papers (21.69%). 4) Finally, in terms of lexical density, the ratio of function words to content words was similar across students' research papers (57.59%), publications in national journals (57.19%), and publications in international journals (57.52%). Students who are required to write research papers and novice researchers are recommended to adhere to the standardized ratios set by publications in international journals when aiming to publish their research in such journals.