This study aimed to compare the readability levels of the reading passages in the General Aptitude Test (GAT) and those of English textbooks for the Upper Secondary Level in Thailand. The corpus of the General Aptitude Tests (C-GAT) was compiled from 70 reading passages on GAT test papers from 2009-2021, and the corpus of English textbooks (C-ET) was compiled from 66 English textbook series. The data were quantitatively analyzed to investigate the readability levels of the reading passages in the two sets of corpora. The instrument of this study was Coh-Metrix--a common core text ease and readability assessor. The data were analyzed based on 11 categories of language characteristics divided into 106 indices. The results showed that the reading passages in the C-ET were easier than those in the C-GAT in 10 out of 11 categories with significant mean values in 4 categories: text easability principal component scores, referential cohesion, connectives, and word information. As a result, it can be concluded that the readability levels of the teaching and testing materials were incongruent. Thus, English textbook designers, English teachers, and test developers need to concentrate on the relationship among reading passages used in both national tests and English textbooks in order to achieve the right balance between students' English reading skills and results of the tests.