Ectopically expressed miR-181d inhibited renal fibrosis in vivo and in vitro, resulting in a decline in the renal fibrosis-related factors α-SMA, Col4α1, TGF-βR1, and collagen I. miR-181d was consistently reduced in both the serum of patients with renal fibrosis and in the kidneys of UUO rats. Here, COL4A1 is linked to renal fibrosis.