Assessment has several advantages like involving students in learning and making them cognizant of their strengths and weaknesses. The effects of peer assessment and self-assessment as two main kinds of assessment have not examined on EFL learners' self-regulated learning, critical thinking, and problem-solving skills; therefore, the present research attempted to inspect the impacts of self and peer assessment activities on self-regulated learning, critical thinking, and problem-solving skills of Iranian EFL learners. To do so, 75 Iranian EFL learners were selected and divided into two experimental groups (peer assessment group (PAG) and self-assessment group (SAG)) and one control group (CG). Then, three questionnaires of self-regulated learning, critical thinking, and problem-solving skills were administered to the students as the pre-tests of the study. After that, one experimental group received the treatment via using self-assessment and the other experimental group received the instruction through peer assessment. The control group received a teacher-based assessment instruction. After teaching 13 English conversations to all groups, 3 post-tests were given to them to measure the impacts of the treatment on their self-regulated learning, critical thinking, and problem-solving skills. The results of one-way ANOVA showed that there was a substantial difference between the post-tests of the experimental groups and the control group. The results showed that both experimental groups outperformed the control group in the post-tests of self-regulated learning, critical thinking, and problem-solving skills. Furthermore, the outcomes revealed that there were no significant differences between the post-tests of the experimental groups. It can be concluded that both types of assessments play a vital role in English language teaching and learning. Based on the results of this study, language teachers, materials developers, and education authorities can concentrate on using self and peer assessments as one of the main strategies to develop the language learning ability of Iranian EFL learners.