The essential function of critical thinking in education is obvious by many studies done in this field. The main purpose of this article is to find the relationship between critical thinking levels of Iranian EFL learners and their performance on different modes of writing. The sample of the study selected among those who studying English at the advanced level at Kish Institute of Science and Technology, Rasht, Iran. The instruments used in this study included the Longman paper and pencil test (2004) for ensuring the homogeneity of the learners, a critical thinking questionnaire (Honey, 2004) with 30 items into the 5-point Likert scale type that used to divide learners into high and low critical thinkers and an analytic scale of Weir (1990) for assessing participants' argumentative and descriptive writings. The study followed an ex-post facto design. The results of inferential statistics revealed a statistically significant difference between the writing performance of high and low critical thinkers in both the descriptive and argumentative modes. It was found that the high critical thinkers' writing was better in both modes of writing compared to the low critical thinkers. The result of this research helps teachers to consider the effect of critical thinking on the learning process. Moreover, the syllabus designers and course-book writers should think about critical thinking as an influential element in their program.