It is different from most previously related studies where Intralingual writing errors from various writing genres, topics, made by learners at diverse levels of education from different foreign language centers, classes, universities and unlevelled sample population have been analyzed. This paper analyzes Interlingual writing style errors in 44 IELTS-formatted opinion essays of the same genre, on the same topic of environment pollution, committed by one Vietnamese class of 44 second year English major students (of the two equal number groups of students: 22 students in each group) at Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam. The qualitative approach has been deployed to ensure the validity and reliability. The findings show the two groups of subjects under investigation have made different kinds of Interlingual writing style errors among which the prevailing errors were Gender-/Opinion-biased, Overemphasis/Exaggeration, Repetition of Words or/and Phrases, and Spoken /Letter writing style. Noticeably, all these students have committed Compound or Integrated errors of two or more of the above single errors due to the deep interference of Vietnamese as their native language and Asian language as the regional language, resulted from the lack of essay writing style knowledge in English mainly because of the learners’ humble motivation and the teaching and learning environment as well. 
From these findings, some implementations related to the students’ motivation in learning Writing have been put forward, followed by the University’s sources of Writing materials and English seminar, the teachers’ essay writing teaching method and their attitudes of writing style error correction. These solutions could help students of English and those Vietnamese students of English major at Nguyen Tat Thanh University improve their English writing style, write their English opinion essays better, avoid Interlingual writing style errors and do the IELTS- formatted argumentative essays successfully, which hardly any research has done.