The present study aimed at assessing the level of the readability of the 11th gradetexts biology book in the Sultanate of Oman and its relation to the academic achievement as well as its relation to the gender of the students. To achieve the objectives of the study, the researchers used the descriptive approach in their study. The Cloze test was prepared for eight texts from the biology book to measure the level of its readability, distributed 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) males, and (48) females. 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 in the above level is 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. Finally, the results showed that the level of female students’ achievement is higher than that of males. In light of the results, the researchers recommended a number of recommendations, including: To take care of measuring the readability of the books of different subjects, and verify their relevance to the level of the class or stage prepared for them, and verify their suitability to the level of the class or stage prepared for them, through the procedure of testing and assessing the readability of its texts before generalization.