Readability has long been regarded as a significant aspect in English language teaching as it provides the overall picture of a text's difficulty level, especially in the context of teaching and testing. Readability is a practical consideration when making decisions on materials to match a text with target readers' proficiency. However, few studies have compared the readability levels of teaching and testing materials in terms of the difficulty of passages. The present study, therefore, aims to explore the readability levels and the linguistic characteristics of reading passages in English textbooks and the Thai National Education English Test based on three readability formulas and eight aspects of linguistic characteristics as provided by the Coh-Metrix computational tool. Two sets of corpora were generated and analyzed by using Coh-Metrix as the main instrument. The obtained data from the reading passages compiled in the English textbooks and the Thai national education English test were compared to explore the significant differences. The results revealed a mismatch in the readability levels and linguistic characteristics. Passages from the English textbooks are easier than those used in the English test. It is recommended that all stakeholders in both teaching and testing administration be aware of the different levels of readability between reading passages. More considerations should be made when preparing the teaching and testing materials because a suitable difficulty level will ensure that students receive the most benefit from the materials. Moreover, an incongruity could affect students' learning and testing performance.