This study aimed to investigate the extent of inclusion of the dimensions of the nature of science in grades eleven and twelve biology textbooks in The Sultanate of Oman and students' understanding the nature of science. To achieve the study objectives, the researcher designed two study instruments: a content analysis card and a test for the understanding nature of science. To check validity of the two instruments, they were forwarded to a number of educational specialists. The reliability of the analysis card was established through the calculation of the reliability coefficient among analysts and was (94.12%), while the reliability of the test was established using the Cronbach Alpha equation and was (0.78). The results revealed uneven percentages of the extent of inclusion of the dimensions of the nature of science as it reached (45.50 %) in grade eleven's book and (69.19 %) in grade twelve's. The results revealed the inclusion of all dimensions of the nature of science in the contents of both biology books but the results were uneven. The results also revealed a level of students' understanding the nature of science at (58.61 %) which is below the educationally acceptable level (80%) and the existence of significant differences of the significance level (a =0.05) on the level of students' understanding of the nature of science due to the sex variable in favor of the female students. The study recommended that the dimensions of the nature of science should be included more in the content of the two biology books and in a more balanced manner, and that the level of students' understanding the nature of science be raised.