BACKGROUND Studies have reported that the integrated analysis of microRNA (miRNA)-messenger RNA (mRNA) expression is valuable in exploring gene regulation systemically. OBJECTIVES The objectives of this study were to identify miRNAs and genes involved in atrial fibrillation and to explore the mechanisms underlying atrial fibrosis. METHODS We used microarrays to compare the differences in both miRNA and mRNA expression profiles in the left atrial appendage of patients with nonvalvular paroxysmal atrial fibrillation and healthy controls. Furthermore, the quantitative real-time polymerase chain reaction was used to confirm the reliability of the microarray data, prediction of the adopted databases, and Ingenuity Pathway Analysis of miRNA-mRNA expression in order to identify the miRNA target genes, examine the functions and pathways in which the target genes are involved, and construct an miRNA-target gene regulatory network. We further investigated the roles of miRNA-146b-5p in the mechanisms of atrial fibrosis. RESULTS We identified 10 differentially expressed miRNAs and 624 differentially expressed mRNAs, among which only 1 miRNA-target gene pair miR-146b-5p and tissue inhibitor of metalloproteinase 4 (TIMP-4) were constructed. The validated results revealed that miR-1466-5p, matrix metallopeptidase 9, and collagen content were upregulated whereas TIMP-4 was downregulated in patients with atrial fibrillation. After the transfection of miR-146b-5p into cardiac fibroblasts, TIMP-4 expression was markedly reduced and collagen content was increased. Moreover, Luciferase results confirmed that TIMP-4 was a target of miR-146b-5p. CONCLUSION The identified miRNA and mRNA may represent a potentially novel molecular regulatory network, which may provide a better understanding of the molecular basis of remodeling in atrial fibrillation. miR-1466-5p probably acts as an intracellular mediator in the maladaptive remodeling in atrial fibrosis in atrial fibrillation.