As assessment plays an important role in the process of teaching and learning, this research explored the impacts of formative and summative assessments on academic motivation, attitude toward learning, test anxiety, and self-regulation skill of EFL students in Iran. To fulfill the objectives of this research, 72 Iranian EFL learners were chosen based on the convenience sampling method assigned to two experimental groups (summative group and formative group) and a control group. Then, the groups took the pre-tests of test anxiety, motivation, and self-regulation skill. Then, one experimental group was trained by following the rules of the formative assessment and the other experimental group was taught according to the summative assessment. The control group was instructed without using any preplanned assessment. After a 15-session treatment, the post-tests of the test anxiety, motivation, and self-regulation skill were administered to all groups to assess the impacts of the instruction on their language achievement. Lastly, a questionnaire of attitude was administered to both experimental groups to examine their attitudes towards the impacts of formative and summative assessment on their English learning improvement. The outcomes of one-way ANOVA and Bonferroni tests revealed that both summative and formative assessments were effective but the formative one was more effective on academic motivation, test anxiety, and self-regulation skill. The findings of one sample t-test indicated that the participants had positive attitudes towards summative and formative assessments. Based on the results, it can be concluded that formative assessment is an essential part of teaching that should be used in EFL instructional contexts. The implications of this study can help students to detect their own weaknesses and target areas that need more effort and work.