Traditional virus-induced gene silencing (VIGS) is a powerful virus-based short interfering RNA-mediated RNA silencing technique for plant functional genomics. Besides short interfering RNAs, microRNAs (miRNAs) have also been shown to regulate gene expression by RNA silencing in various organisms. However, plant virus-based miRNA silencing has not been reported. In addition, a number of plant miRNAs have been identified or predicted, while their functions are largely unknown. Thus, there is an urgent need for the development of new technologies to study miRNA function. Here, we report that a modified cabbage leaf-curl geminivirus vector can be used to express artificial and endogenous miRNAs in plants. Using this viral miRNA expression system, we demonstrate that VIGS using artificial miRNAs, dubbed as "MIR VIGS," was effective to silence the expression of endogenous genes, including PDS, Su, CLA1, and SGT1, in Nicotiana benthamiana. Silencing of SGT1 led to the loss of N-mediated resistance to Tobacco mosaic virus. Furthermore, using this viral miRNA expression system, we found that viral ectopic expression of endogenous miR156 and miR165 but not their mutants in N. benthamiana resulted in earlier abnormal developmental phenotypes, and expression of miR165 induced abnormal chlorotic spots on leaves. These results demonstrate that the cabbage leaf-curl geminivirus-based miRNA expression system can be utilized not only to specifically silence genes involved in general metabolism and defense but also to investigate the function of endogenous miRNAs in plants.