Micro RNAs are 21-24 nt long RNA molecules involved in the regulation of gene expression on mRNA level. Mature miRNA molecules associated with the RNA-induced silencing complex (RISC) guide specifically mRNA cleavage or inhibit the translation. In plants selective mRNA degradation in miRNA-dependent pathway is a widespread mechanism that occurs more frequently than translation inhibition. miRNA precursors, known as pri-miRNA, are processed in several steps to produce mature miRNA molecules. There are several plant proteins involved in miRNA processing: DCL1, HYL1, SE, HEN1 and HASTY. The role of these proteins is discussed.