The present study takes Engeström human activity system model (1987, 1999) as a theoretical framework to investigate the CLT-based English curriculum reform in Iran which was initiated in 2013 by the Ministry of Education in public schools. With the premise that human activity is artifact-mediated and goal-directed, activity theory makes it possible to demonstrate the complex and dynamic relationship between various institutional, social, and individual factors by revealing different contradictions that language teachers would experience as they attempt to implement CLT in their classroom contexts.  In addition to 23 language teachers who consisted the main participants of this study, three other groups including 17 teacher directors, 23 students, and 20 parents took part. Document analysis, semi-structured interview, and classroom observation comprised the data collection instruments. The results indicated that despite their optimism about and keen interest in CLT-based reform, Iranian language teachers could not successfully implement CLT due to their inability to successfully resolve the contradictions that emerged in their activity system. These contradictions in turn stemmed from a number of difficulties that emanate from various sources including teachers themselves, students, their parents, school staff, educational system, and the new package. Based on the findings, a number of implications and suggestions are provided for the Iranian curriculum developers, language teachers, parents, and teacher directors.