The study aimed to assessing the level of the readability of the 11th grade biology book in the Sultanate of Oman and its relation to the academic achievement and the attitudes of the students towards biology, as well as its relation to the gender of the students. To achieve the objectives of the study, the researcher used the descriptive approach in his study. The Cloze test was prepared for eight texts from the biology book to measure the level of its readability. The researcher also prepared a scale of the students' attitudes towards biology, distributed both on a randomly selected sample of )94( students from the 11th grade of two public schools affiliated to the Directorate of Education in Muscat Governorate, of which(46) male, and (48) female. To determine the level of student achievement, the researcher relied on their results at the end of the first semester of biology The study found the following results: The level of the readability of the biology book for the 11th grade is suitable for students in this educational stage. The percentage of students over disappointed level was 74.2%, which is the total average of students in the independent and educational levels. It also showed that the level of female students 'readability of texts is higher than that of male students, where there were statistically significant differences in the students' grades on the cloze test and for the benefit of the students. On the other hand, there was a strong correlation between the average readability of all texts for all students and the average achievement of all students. As for the readability and its relation to the students' attitudes, it was found that there was a medium correlation between the average grade of all texts for all students and the average degree of attitudes towards biology for all students. In the light of the results, the researcher recommended the importance of measuring the readability of the books of the various subjects, and checking their suitability to the level of the classes or stages prepared for them, through the experience and measurement of the text of its texts before circulation.