Recent studies have revealed associations between single nucleotide polymorphisms (SNPs) in microRNA (miRNA) genes and diseases. However, association studies to decipher the interactions between miRNAs and their target genes remain to be conducted. Here, we investigated the association of growth and wood traits with SNPs in Pt-miR397a and its targets, in 261 individuals from a natural population of Populus tomentosa. Of the 57 SNPs identified in Pt-miR397a, three strongly affect its secondary stability, and SNPs in target sites in Pt-LAC20 and Pt-HSP40 changed the binding affinity of Pt-miR397a. Single-SNP association analysis revealed that SNPs in Pt-miR397a significantly associated with α-cellulose content and stem volume, and SNPs in target genes also associated with growth and wood-property traits. Multi-SNP association analysis with additive and dominant models found that SNPs in six potential target genes associated with at least one trait in common with Pt-miR397a, revealing a possible genetic interaction between Pt-miR397a and its targets. Furthermore, epistasis analysis revealed epistatic interactions between SNPs in Pt-miR397a and its target genes. Thus, our study indicated that SNPs in Pt-miR397a and six target genes affect wood formation and that association studies can reveal the interactions between miRNAs and their target genes.