Giving students written feedback has been a common practice in L2 writing instruction. Written feedback has been widely studied in second language writing and acquisition research, yet many questions and disputes remain concerning its effectiveness (See Ferris, 2010). While most research on written feedback has adopted the cognitive psychological perspective focusing on its effectiveness (Hyland, 1998, 2000), this study aims at discovering the mediation, mutual growth and engagement between tutors and writers in an asynchronous online writing tutorial. In the tutorial, U.S.-based tutors (teacher candidates in a teacher education program) worked with L2 writers (graduate students) in Taiwan on their English academic writing course assignments (biodata and summary). Data sources included written comments by the teacher candidates, writer's drafts, uptake documents, interview transcripts, self-evaluations, and field observation notes. Oriented by speech act and Vygotskian theoretical framework and using discourse analysis, this qualitative case study identified 12 feedback acts under three categories (direct, indirect, and conversational Feedback Acts) among three focal dyads throughout the tutorial. Findings suggest that the three tutors used feedback acts strategically to guide the L2 writers, particularly using IFA and CFA as mediational tools to provide various metalinguistic explanations, give extended information, and asked thought-provoking questions to stimulate writers' thinking in the tutorial process along with the corrections, suggestions, or requests they made. As writers incorporated more than 70% of the feedback, they found the tutorial process beneficial for their revision and learning of English academic writing. Tutors also learned to accommodate writers' needs, providing feedback within their zone of proximal development and applying concept-based instruction and dynamic assessment. This study contributes to second language writing and learning research, revealing the complexity of tutor-writer interaction and feedback process and providing a window into how written feedback can foster communication and dialogues between tutors and writers. Close examination of discourse in the tutorial context offers insights into the mutual growth and engagement for the participating teacher candidates and L2 writers. This study also has implications for both L2 writing instructors and teacher educators who seek new ways to engage language learners and teacher candidates in their learning processes.