By adopting the notion of Error Analysis (EA) and the Surface Strategy Taxonomy as the theoretical frameworks, this study investigates the types of grammatical errors produced by the students in their argumentative writing. Furthermore, the sources behind the errors have also been analyzed. This study employed the qualitative method design in which the case study approach was utilized. Ellis’ five-step procedure of EA was adopted to analyze the data of essays comprising of 150-300 words written by 23 EFL students of the fourth-semester at English Department of Islamic State University (UIN) Ar-Raniry Aceh. They were purposively selected as the participants of this study. Based on the findings, it was found that omission was the most frequent error identified in the students’ writings. Overall, 122 cases out of 195 cases were categorized under this type of error which constitutes of 63%. The number-marker, verb-tense, article, preposition, subject-verb agreement, and pronoun were the categories of the frequent errors made by the students, respectively. It was followed by addition (18%), misformation (15%), and misordering (5%). Significance to the sources of errors, intralingual transfer turned out to be the main reason that provoked the blunder in the students’ writings. It was apparent that some of the interlinguistic contrast was the reasons behind the errors. It appeared that the diverse systematical concept between the Indonesian language and English in terms of verb conjugation factor, inflectional morpheme, and auxiliary-verb abandonment were the strong contenders of the error sources. Additionally, interlingual transfer and context of learning also took part for the reason behind the error.