This study investigated grammatical difficulties encountered by students of the People’s Police University while learning grammar in ESP courses. The design included questionnaires to teachers and students, and classroom participant observations. The findings indicated that the student respondents only perceived four target features to be difficult to learn. The results also showed that the students’ perceptions of grammatical difficulty were influenced by several factors including the inherent complexity of rules, L1 transfer, students’ differences in language aptitude, students’ infrequency of practice, and teaching methodology. The findings will hopefully be useful for teachers and researchers with an interest in this line of research.