Among the factors influencing students' educational performance of English language course, critical thinking is thought to be of crucial importance. Thus, the present study set out to explore the relationship between critical thinking, its subscales and English language achievement. Also, the predictability of English language proficiency based on critical thinking was investigated. The statistical population included all eleventh grade female students in district one of Zanjan city in the academic year of 2018-2019 with1400 people. The statistical sample was 300 people based on Cochran's formula. A multi-stage random sampling method was used to select the participants. The California Critical Thinking Skills Test, Form B was used to assess students' critical thinking ability. The participants' scores of English language course at the end of the first semester were considered as a criterion for their English language proficiency. The results of the correlation analyses revealed that critical thinking and its subscales correlated significantly with academic achievement (p=0.0). Also, based on regression tests, it was revealed that English language proficiency is predictable based on critical thinking. The findings of this study would be beneficial for students, teachers, schools and educational officials.