This mixed-methods study explored Iranian English teachers' perceptions of barriers to implementing an innovative CLT-based curriculum in Iranian high school English classes. The sample of the study was composed of three groups: 23 English teachers who participated in semi-structured interviews, 217 English teachers who completed the initial version of the questionnaire for construct validation, and 143 English teachers, including 45 Teaching English as a Foreign Language (TEFL) graduates, 58 beginning teachers, and 40 experienced teachers, who completed the validated questionnaire. The instruments utilized were a semi-structured interview and questionnaire. The thematic analysis of the qualitative data indicated that the barriers were of five core categories related to institutions, culture, learners, teachers, and CLT. The analysis of the quantitative data demonstrated that on the continuum of full/non-implementation, the participants stood considerably away from the full implementation extreme believing that few CLT principles could be implemented. The findings also revealed that the graduates' perceptions of barriers were significantly different from those of beginning teachers and experienced teachers, while no significant difference was observed between the beginning teachers' perceptions and those held by the experienced teachers. It is concluded that there is a huge gap between the curriculum developers' intentions and teachers’ perceptions, both in urgent need of modifications.