Assessment is an inseparable part of teaching and learning, and it helps teachers and students to modify their teaching and learning processes. One type of assessment is peer assessment (PA), and its effects were examined on developing Iranian EFL learners' reading comprehension (RC), reading motivation (RM) and vocabulary learning (VL) in this research. To achieve this goal, 60 Iranian EFL students at the intermediate level and with the age range of 18 to 26 years were selected based on a convenience sampling method and divided into two groups: the control group (CG) and experimental group (EG). After that, the groups took the pre-tests of RC, RM, and VL. The EG then was divided into six sub-groups of five, and PA was applied to evaluate their reading and vocabulary performances. The learners in the EG checked their classmates' performances in peers with the teacher's help. In the CG class, the participants themselves assessed their own performances after each test with the help of the teacher. After a 15-session treatment, the post-tests of vocabulary, RM, and RC were carried out on both groups. The outcomes of the one-way ANCOVA tests demonstrated that the EG outflanked the CG on the three post-tests of RC, RM, and VL. In fact, the results indicated that using PA generated positive effects on Iranian EFL learners' RC, RM, and VL. It can be claimed that the PA is a practical technique to improve EFL learners' language learning. The implications of this research can reduce students' dependency on teachers and increase their independence in the evaluation process.