This study is aimed to reveal the types of grammatical errors made by students in speaking English in the English department of IAIN Takengon. When speaking, students generally did not realize and understand the classification of errors they were making. Thus, it is deemed necessary to identify, classify, and describe the various grammatical errors as an attempt to build their awareness of using structured and standardized English rules. This research was conducted using a qualitative descriptive method. After analyzing the errors proposed by Dulay, the findings showed that the most common error in students’ grammar is misuse or misplacing of verbs, nouns, conjunctions, pronouns, prepositions, and parts of speech. Based on the error category, students' most common error is omission, followed by overgeneralization, over-regulation, and misordering, completed by other errors such as carelessness. Meanwhile, the causes of grammatical errors were the internal factors; anxiety, limited linguistic resources, and motivation deficiency; the external factors were an unsupportive environment and the stressful teaching and learning process.