English reading skill is one of the most significant skills that most Thai students are struggling with in Thai national English proficiency tests, i.e., GAT, O-NET, 9 Common Subjects. Although many studies have focused on Thai students’ reading comprehension in Thai school standard curriculum, little attention has been paid to the Thai national English proficiency tests. Therefore, this study aims to investigate difficulties that Thai high school students confront on the Thai national English proficiency test by focusing only on the GAT ENGLISH (General Aptitude Test) since this test is required by many faculties in Thai university entrance exams. Data were collected from nine high school students via test scores, observations, and focus group interviews. The participants were asked to do the test, which was the reading part of GAT English. After the test session, all participants were interviewed about the issues they found during the test. The findings show that reading strategies are the most significant difficulty followed by vocabulary, reading comprehension, grammar, choice confusion, limited time examination, and background knowledge respectively.