The aim of this study is to examine the readability levels of the texts on the biology units in the science textbook prepared for the sixth grade. This study, which is based on a qualitative approach, was carried out with the method of document analysis. The textbook examined in the study is Middle School and Imam Hatip Middle School Science Textbook 6, which has been used since the 2018-2019 academic year. Regarding biology topics, the textbook includes in two units (Unit-1: Our Body Systems, Unit 2: Our Body Systems and Health). In the study, the learning outcomes of the units were taken as a basis in the selection of the texts to be analyzed in the textbook. In the study, a total of 22 texts were randomly selected by taking into account the learning outcomes in the curriculum. In addition, it has been paid attention that each text contains at least 100 words. Atesman readability formula was used to determine the readability levels of the selected texts. According to the findings obtained as a result of the analysis, it was determined that the readability level of the texts for unit-2 was medium, while the readability level of the texts for unit-6 was found to be difficult. In the light of the findings, suggestions were made that the textbooks should be prepared in accordance with the student level in terms of readability and that texts consisting of simpler and shorter sentences should be used more.