This study examines grammatical errors in student essays, with the aim of identifying and categorizing those errors. The four grammatical categories analysed in this study include omission, addition, mis-formation, and mis-ordering. A quantitative research method was employed to gather and analyse data from the English Writing Class of Fajar University (Unifa) group on Facebook. The analysis was conducted using the grammar theory proposed by Heidi Dulay (1982). The findings of this study indicate that the majority of grammatical errors in the selected essays were related to omission as 43% errors. Mis-formation accounted for 41% of errors, addition constituted 12% of errors, and mis-ordering represented 4% of errors. This study provides valuable insights into the prevalent grammatical errors in student essays, highlighting areas where further attention and improvement are needed both in Grammar and Writing class.