The aims of this study are to investigate the difference between peer- and teacher-assessment and to identify and explore the components in given criteria that students find easy or difficult when writing essays. One hundred and four essays and a survey were collected from 26 students at a Korean university in Seoul. The essays were analyzed to determine the difference between teacher-and peer-assessment and to evaluate the effects of peer-assessment. The results report that the teacher gave students lower scores in the first essay than students did as peer-assessment. However, the teacher gave students higher scores in the second essay than students did, which helps students have confidence to write English composition. Several students thought that the grammatical accuracy was the most difficult since it is not easy to accurately write their essays as English as foreign language (EFL) learners. Meanwhile, the survey results revealed that more than half the students thought that exchanging peer-assessment (69%) and using criteria (62%) were of much or some help in developing their essays. In this respect, conducting peer-assessment is helpful for students to monitor their work, to evaluate their partner's work and to develop second/foreign language (L2) writing.