The importance of grammatical knowledge in the writing process, this research aimed to describe the types of grammatical errors, to examine the most frequent grammatical errors, and to find the sources of grammatical errors on writings of students who have the highest writing score of English Education Department in Universitas Islam Malang. The design of this research was descriptive qualitative to analyze the learners' writing. The data was collected from the learners writing of the second semester of the English Department in Universitas Islam Malang in 2020-2021. The key instrument which was used in this research was the researcher. The researcher also used a supporting instrument; the instruments used in this research are a table of error analysis and Grammarly application. Using Dulay’s theory that is Surface Strategy Taxonomy becomes the main key in the data analysis. From the 15 data of learners' writing, the researcher found four types of grammatical errors that matched Dulay's theory. The four types of errors are Omission error was found 39 times, misformation error was found 32 times, addition error found 50 times, and misordering errors 2 times in the analysis. As for the sources of errors that learners made, intralingual transfer was found 95 errors, and Interlingual transfer was found 42 errors. Based on data findings, it was concluded that learners were still influenced by the first language or the influence of mother tongue, and learners had difficulties learning grammatical structures in their writing process. Therefore, learners and teachers need to pay attention to the most frequent errors in paragraph writings, such as tenses structure, grammar, and application in writing. If these errors can be corrected, it can minimize learners’ errors in writing. Keywords: Grammatical Error, Error Analysis, Writing Skill