In this study, the researcher used the many-facet Rasch measurement model (MFRM) to detect two pervasive rater errors among peer-assessors rating EFL essays. The researcher also compared the ratings of peer-assessors to those of teacher assessors to gain a clearer understanding of the ratings of peer-assessors. To that end, the researcher used a fully crossed design in which all peer-assessors rated all the essays MA students enrolled in two Advanced Writing classes in two private universities in Iran wrote. The peer-assessors used a 6-point analytic rating scale to evaluate the essays on 15 assessment criteria. The results of Facets analyses showed that, as a group, peer-assessors did not show central tendency effect and halo effect; however, individual peer-assessors showed varying degrees of central tendency effect and halo effect. Further, the ratings of peer-assessors and those of teacher assessors were not statistically significantly different
