MicroRNAs (miRNAs) are endogenously encoded small RNAs that post-transcriptionally regulate gene expression and play essential roles in plant development, growth, and stress response.Currently, little information on the transcriptome of miRNAs is available in Betula luminifera, one of the economically important timber tree mainly distributed in Southem China.Recent advances in sequencing technologies have allowed the identification of conserved and novel miRNAs in many plant species.Here, we used high-throughput sequencing technologies to identify and characterize the miRNAs during early stage of tension wood formation in Betula luminifera.Three small RNA libraries were constructed for deep sequencing from normal and tension woods (TW6h and TW48h with tension treatment for 6 and 48 h, respectively).135 miRNAs belonging to 45 known miRNA families and 7 novel miRNAs were identified, and four different expression patterns of differentially expressed miRNAs were also revealed.Among these differentially expressed miRNAs, some miRNAs related to hormone response and stress stimulus were further validated by qRT-PCR.Moreover, computational analysis was carried out to uncover potential targets of the known and novel miRNAs.Predicted target genes included not only transcription factors but also genes involved in various biological processes including stress responses, primary and secondary metabolism.Potential targets such as HD-zip Ⅲ and SPL transcription factors were verified by RLM-RACE.This study provides a basic catalog of miRNAs and their targets in Betula luminifera to facilitate future improvement and exploitation of the timber tree.