This study aimed 1) to explore Iranian EFL teachers’ attitudes towards the different aspects of the English textbooks prospect used at the first three grades of high school in Iran 2) to probe any relationship between the teachers 'attitudes towards prospect series and their TKT level. A questionnaire and a structured interview were utilized as instruments in this research. The questionnaire was structured in 3 units: The first unit was intended to record the respondents’ demographic information such as their age, sex, field of study, university, and teaching experience; the second unit including 24 items was intended to discover the respondents’ level of TKT; and the third unit including 28 items was designed to elicit the teachers’ attitudes towards the current high school textbooks. This questionnaire of attitude was designed in the form of Likert scale including strongly disagree, disagree, slightly disagree, agree, slightly agree and strongly agree. The data of the study were collected through administering the questionnaire to 80 teachers from Amol, Isfahan and Tehran. Also, semi-organized interviews have been carried out with twelve teachers selected from the eighty to cross-check the results of the study. Due to the rate of the absence, seventy of them completed the questionnaire. After administering questionnaire, the data were analyzed descriptively and inferentially. Out of the total seventy participants, Twenty five of them scored 3 in their TKT. Out of these 25 participants, two of them were intermediate achievers in the attitudinal questionnaire. Eighteen of them were high achievers and five of them were very high achievers. Also, forty five participants scored 4 in their TKT. Out of these 45 participants, one of them was intermediate achiever, thirty five of them were high achievers and finally nine of them were very high achievers. In order to measure whether Iranian EFL Teachers' attitudes towards the Book series ''Prospect'' taught in Iran's Schools and their TKT are related to each other, the researchers used Pearson Chi 2 and Fisher's Exact and the Pearson chi 2 p-value was 0.516 and the Fisher's exact p-value was 0.642. Since they are above %0.05, we can conclude that there is no significant relationship between Iranian EFL Teachers' attitudes towards the Book series ''Prospect'' taught in Iran's Schools and their TKT. The analytic study on the scores of the participants revealed the facts that a) the participants had positive (high) attitudes towards the English series Prospect b) There is not any significant relationship between the teachers’ positive attitudes towards the new text books and their TKT level.Keywords: Iranian, EFL Teachers, Attitude, Textbooks, High School, TKT