English for Specific Purposes (ESP) students assume that the speaking ability is the most challenging skill to learn English because the English learning class and the environment are unsupportive to the students to speak in English. Communication is the most important facet of language learning both in EGP (English for General Purposes) and ESP contexts to realize how much anxiety can impact language learners' communication. This study aimed at comparing the impact of self- and peer-assessment on ESP Students’ speaking ability and speaking anxiety. In order to carry out this study, 60 second-semester students of psychology were chosen among 90 students from Azad University of Zanjan to participate in this study using quasi-experimental and comparative design. These participants were divided into experimental groups A and B. Both groups took PET (Preliminary English Test) speaking test and speaking anxiety questionnaire at the outset of study. Then, the treatments were embarked upon by the two groups: experimental Group A received self-assessment speaking instruction while the second group exercised peer-assessment speaking instruction. The course took eight weeks. Afterwards, PET speaking test and speaking anxiety questionnaire were again given to the two groups. Finally, the scores gained on these two instruments, on both groups and on pre- and post-tests were analyzed and compared. ANCOVA (F(1, 57) = 12.43, p = .003) revealed that self-assessment is more effective than peer-assessment to improve speaking ability and reduce speaking anxiety of ESP students. This study has pedagogical implications for EAP and ESP instructors in implementation of both self- and peer-assessment to teach speaking.