Although the importance of summary writing is well documented in prior studies, few have investigated the evaluation of written summaries. Due to the complex nature of L2 summary writing, which requires one to read the original material and summarize its content in the L2, raters often emphasize different features when judging the quality of L2 summaries. Therefore, this study examines the ratings of English-language summaries written by Japanese university students in order to identify differences in EFL instructors' evaluations. Fifty-one Japanese EFL university students read a passage and then wrote an English summary without receiving any instructions concerning summary composition. The raters included three native English speakers (NESs) and three non-native English speakers (NNESs), who individually evaluated each summary using the Educational Testing Service's holistic rubric. Analysis of inter-rater reliability revealed a lower Cronbach's alpha coefficient for NNES raters (a = 0.39) when compared to NES raters (a = 0.77). Comments were collected from raters regarding the difficulty of evaluating summaries, and the causes of such difficulties were examined. Comments from NNES raters more concerned vocabulary use and paraphrasing, whereas the NES raters concentrated on content and language. This study also explores ways to potentially improve the holistic rubric by examining feedback from raters regarding their rating experiences.