MicroRNA (miRNA) is a group of recently discovered small RNA molecules that have a significant role in regulating gene and protein expression. These small RNAs (-22 nt) can bind to 3′- untranslated regions (3′-UTRs) and induce mRNA degradation and repress protein expression. They play important roles in development and maintenance of normal cell function. More recently, a few studies have suggested that miRNA may also serve as an important regulator of circadian rhythmicity, which casts new lights on our understanding of biological clocks. Here we describe the function of miRNA in regulation of circadian system.