The educational context in Malaysia demands students to be equipped with sound grammar so that they can produce good essays in the examination. However, despite having learnt English in primary and secondary schools, students in the higher learning institutions tend to make some grammatical errors in their writing. This study presents the grammatical errors made by tertiary students in their writing. The participants were a group of Diploma students who sat for a university entrance exam. One hundred and forty-four written essays of the students were collected and analysed using content analysis. Findings revealed that subject-verb agreement and tenses were the most common type of errors. Students over-generalised and perceived that the tenses could be used interchangeably. Another common error found was in the students' construction of complex sentence. In such constructions, they failed to include essential and nonessential clauses. If teachers do not teach strategies to assist students in comprehending the concept of Subject-Verb Agreement (SVA), tenses, essential and nonessential clauses, these students will continue to make such errors in their tertiary education. The findings may have useful implications for English language teachers as understanding students' learning difficulties and providing appropriate grammar instruction is the key to effective teaching for ESL teachers.