The field of ELT is constantly witnessing the introduction of new instructional approaches: one such perhaps recent initiative is critical discourse analysis (CDA). Accordingly, the present study was an attempt to investigate the impact of CDA instruction on Iranian EFL learners’ descriptive and argumentative writing ability. To fulfill the aforementioned purpose, a sample TOEFL was primarily piloted among a group of 30 upper intermediate EFL learners by the researchers; with the acceptable reliability and item analysis indices achieved, then the researchers administered the test among another group of 90 upper intermediate learners. Ultimately, those 60 learners whose scores fell one standard deviation above and below the mean were chosen as the participants of the study and were randomly assigned to a control and an experimental group with 30 participants in each. Both of these groups underwent the same amount of teaching time during 20 sessions which included a treatment of CDA instruction based on Jank’s (2005) set of 14 features for the experimental group. A posttest was administered at the end of the instruction to both groups and their mean scores on the test were compared through a multivariate analysis of variance. The result (F = 14.41 and p = 0.000 < 0.05) led to the rejection of the two null hypotheses raised in this study, thereby demonstrating that the learners in the experimental group benefited significantly more than those in the control group in terms of improving their descriptive and argumentative writing ability. Hence, the major pedagogical implication of this study is that CDA instruction can be effectively used to assist EFL learners improve their argumentative and descriptive writing ability.
