Abstract-Alternative assessments have increasingly gained attention in L2 writing. One of the areas of research which has received much attention in the literature of ELT is the use of self-and peer-assessments and their effects on developing the writing ability of ESL/EFL learners. The purpose of this study was to find the effect of self-and peer-assessments on the argumentative writing performance of intermediate Iranian EFL learners. For this purpose, 36 intermediate EFL students were selected and homogenized based on the results of Oxford Quick Placement Test and an argumentative writing which served as the pre-test of this study. They were then randomly assigned into control and experimental groups to receive different treatments. The compositions of participants in the control group were assessed using traditional teacher-assessment while for the experimental group, self-and peer-assessments were used writing assessment. The results of post-test indicated that the use of self-and peer-assessments significantly affected the writing ability of the learners. Based on the obtained results, it was concluded that using alternative assessments for Iranian EFL students could be helpful in overcoming some of their argumentative writing difficulties. The results of this study have clear implications for both learners and teachers and other stakeholders of ELT. They can use these alternative assessments as a learning opportunity to lower the anxiety and improve the argumentative writing skill of the students.