The aim of this study is to determine the readability levels of the texts in high school Turkish Language and Literature textbooks. The sample of the study, which was carried out as a document analysis in the survey model, was composed of a total of 76 texts, 40 of which were narrative and 36 were informative, taken from the books prepared by the Ministry of National Education Publications for the 9th, 10th, 11th and 12th-grade levels. The readability levels of the texts in the sample were examined according to the readability formulas of Ateşman (1997) and Çetinkaya-Uzun (2010) in the study, which did not include poetry and theatre texts. According to the findings, in the narrative texts analysed based on the Ateşman formula, there were equal numbers of "easy" and "medium difficulty" texts in the 9th grade, whereas "medium difficulty" texts in the 10th grade, and "easy" level texts in the 11th and 12th grade were the majority. It was also determined that the informative texts included more "medium difficulty" texts at all grade levels. According to the ÇetinkayaUzun formula, it was determined that the "educational" level stands out in both narrative and informative texts at all grade levels. When the average word and sentence lengths of the texts were examined, it was found that the informative texts had longer words and sentences than the narrative texts. According to the Ateşman formula, 55% of the narrative texts in the sample were "easy," and 56% of the informative texts were "medium difficulty" level. Additionally, according to the Çetinkaya-Uzun formula, 83% of the narrative texts and 66% of the informative texts were at the "educational" level.