The present case study explores the effects of mediation of dynamic assessment （DA） on EFL writing development of English majors. Two junior English majors and their English writing teacher in a Chinese university were involved in a DA writing task. Student B was asked to write an application letter in English of around 500 words during her spare time. Student A and the teacher, as the mediators, tried to help her improve the essay based on her Zone of Proximal Development （ZPD）. The dialogues in the two mediation processes were recorded and transcribed. Analysis of the data from Student B＇s compositions and the recorded talks during the mediation process showed that there are some similarities between the two mediations including making more use of implicit prompts to guide the learner and playing a positive role to facilitate EFL language growth, especially grammar. The.analysis, however, also showed great differences. The teacher tutor attempted more prompts and more implicit strategies than the peer to show that the teacher could do a better job in exploring the learner＇s ZPD. Even so, the peer might be of important help in improving the language and content before a teacher mediation in the DA process.
