The present study explores the effects of peer intimacy on Korean primary English learners' collaborative group work in terms of writing achievement and peer interaction. Fifty six elementary school students engaged in small group work on English writing tasks. They were grouped in three different types: high intimacy level group, low intimacy level group, and random group. The study results indicated no significant group differences in the students' abilities and attitudes of English writing. However, the three groups differed in the quantity and quality of peer interaction. The high intimacy group produced the largest number of utterance and the most collaborative interactions. The low intimacy group, on the other hand, produced more utterances but less collaborative interactions than did the random group. Overall, the study results suggested that peer intimacy may have no immediate effect on students' learning through small group work but it possibly has delayed effects in prolonged group work by mediating the quantity and quality of peer interactions. Limitations of the study and pedagogical suggestions are provided.