This paper analyzes the intralingual writing style errors in 145 opinion IELTS-formatted essays on the topic of environment pollution under various task types, committed by two Vietnamese classes of second year major English at Nguyen Tat Thanh University, Ho Chi Minh city, Vietnam. The findings show that the subjects of the study made different kinds of intralingual error, which involved immature writing, waffle, confusion, formulaic writing due to the student’s lack of grammatical and lexical knowledge in English. From these findings, some implementations related to the students’ motivation in learning Writing, the teachers’ essay writing teaching method, their attitudes of writing style error correction, the University’s sources of Writing materials and English language scientific workshops have been put forward.