摘要:This research study considered the readability indexes of reading passages in English textbooks taught at Iranian senior high schools. For measuring the readability indexes of the passages, 30 English passages were fed into the computer and Flesch readability indexes of the passages were estimated. This study also examined students’ prior knowledge-interest levels to see to what extent students were interested or had background knowledge of the passages in their English textbooks.120 participants including 60 males and 60 females took part in the study and completed a Likert-type scale questionnaire. Other issues that were analyzed in the study were the association between students’ interest level and their background knowledge level, and the relationship between students’ interest- background knowledge levels with the readability indexes of the passages. The results of the study indicated that the readability indexes of the passages in high school English textbooks did not accord with Flesch readability standard. The results also showed that students mostly had an average and a low level of interest and background knowledge regarding reading passages in their English text books. The findings of the study revealed a significant relationship between students’ interest level and their background knowledge level. This study also showed an insignificant relationship between students’ interest level and the readability indexes of the passages in books two, three and four while based on Flesch readability formula there was a significant relationship between these two variables in book one. The results of the last part of the study revealed an insignificant relationship between students’ background knowledge level and the readability indexes of the passages.