This study aimed at investigating English grammar knowledge of a group of Thai university students. The three main research questions revolved around their knowledge of English grammar, the kinds of difficulties they had encountered in using the grammar as well as their perceptions of the roles of grammar in using English. The participants were administered two types of grammar test: one was the grammar production test (20 items), and the other was the grammar recognition test (40 items). The average scores of the tests were relatively low: 7.30/20 and 21.13/40, respectively. The interview results suggested that the participants viewed grammar as important in learning and using English effectively. They also reported that appropriate teaching of grammar was beneficial as long as the communicative use of the grammar was emphasized. Further, a thorough analysis of sentence structures, which was one of the foci of the lesson learned, helped them to gain a better understanding of English sentences, enabling them to read academic texts more effectively. In short, the participants believed that grammar was essential and was something that was not insurmountable albeit the low scores reported above.