This study presents how EFL learners benefit from the classroom relay writing tasks that involve various simultaneously collaborative interactions. An intact 42-student class of freshmen majoring in accounting is divided into four groups. Each group is assigned with respective relay writing task: with the first sentence available only, or with both the first and last sentences available. The sequential writer of each group either writes while checking the previous single sentence only, or writes while checking all the previous writing as a whole. Each task is accomplished by three steps: independent sentence relay writing, in-group text reading, adjusting and reshaping, and between-group appreciation and evaluation. The composition process is audiotaped and all texts (including both the first and final drafts) are collected. The data showed that 1)the writing turned out to be more vivid and inspiring when the participants only know the first sentence;2)although the writing occurred more complete when the participants write with all the previous writing available, it failed to be much more logic as predicted, indicating the powerful effect of dialogues between sequential writers. Furthermore, by observing the accuracy, consistency and complexity of the written texts, the pedagogical implications of simultaneous in-group and between-group interactions are illustrated.