Scoring rubrics are known to be effective for assessing writing for both testing and classroom teaching purposes. How raters interpret the descriptors in a rubric can significantly impact the subsequent final score, and further, the descriptors may also color a rater's judgment of a student's writing quality. Little is known, however, about how peer raters use a teacher-developed scoring rubric in English as a Foreign Language (EFL) writing contexts. In the current study, Many-Facet Rasch Measurement (MFRM) was applied to examine a scoring rubric for EFL writing and analyze the severity and consistency in rating behaviors between teachers and peer raters. The findings revealed four key points: (1) students' writing skills can be differentiated by this scoring rubric which measured the construct as expected; (2) four rating criteria were reasonably designed, but the scoring bands were not wide enough to separate students' writing ability; (3) teachers were stricter raters than student peers, but both showed a central tendency effect; and (4) teachers had outstanding intra-rater reliability, while peers showed higher inter-rater reliability. The findings provide some evidence for the reliability and internal validity of the scoring rubric and indicate that teachers could use it for assessing EFL writing; however, the scoring bands would need to be broadened to be applied to a broader range of students' writing levels. Implications for introducing scoring rubrics in peer-mediated assessment for teaching writing, developing scoring rubrics in EFL writing assessment, and using MFRM to evaluate scoring rubrics are discussed.