During the last decades various researchers noticed that the traditional approaches to teaching had failed to teach learners to their utmost actual ability, therefore they put great efforts into developing post-modernist approaches and techniques such as critical thinking (CT) and cooperative learning (CL) for improving learning.The present study was an attempt to investigate the comparative effect of practicing CL and CT skills on EFL learners’ writing in a process-based approach to writing on EFL learners’ writing. Sixty Iranian female EFL learners at the intermediate level of English proficiency at Kish Language School were selected among a total number of 90 based on their performance on the Preliminary English Test (PET) and randomly assigned into two groups of CL and CT. Then the researchers administered an argumentative paragraph writing test to ensure the homogeneity of the two groups regarding argumentative writing before the treatment. Both groups were taught the same content through process-based approach throughout the 20-session treatment. Finally, the participants took a paragraph writing posttest including three writing prompts in argumentative genre. The mean scores of the two groups on the posttest were compared through an independent samples t-test. The results led to the rejection of the null hypothesis with the conclusion that CT instruction was significantly more effective than CL in improving EFL learners’ argumentative paragraph writing.
