The study was conducted to assess the common writing errors in Part 2, 3rd Term Final Exam, of students at Thai Nguyen University of Medicine and Pharmacy basing on the criteria of Vietnamese Standardized Test of English Proficiency (VSTEP): Task Fulfillment, Vocabulary, Grammar, and Organization. In the process, the causes of errors were taken into account and some solutions to enhance the quality of students‟ written works were proposed. The qualitative and quantitative method were used in this study, adopting Corder (1973) Steps of error analysis. Data collected from 226 pieces of writing clearly revealed the influence of mother tongue, overgeneralization, and the limitation of sociolinguistics. The findings of this study are highly applicable to the process of teaching and learning Writing skill. The identification and analysis of errors in writing when evaluated in the VSTEP Reference Frame also provide a more comprehensive view for teachers and students during the learning process to reach level B1 and after B1 level.