Objective: MicroRNAs are small noncoding molecules in genome of plants and animals. These molecules have 19-25 nucleotides and regulate gen expression in post transcriptional level (epigenetic mechanism). The number of these molecules is different, however, between 200-100 in primates up to more than 1000 in human.Materials and Methods: MicroRNAs are involved in several processes such as development, cell proliferation, cell death, apoptosis, fat metabolism and cell differentiation. These molecules are responsible for the regulation 30% of expressed genes.Results: MiRNA function done through a similar siRNA complex (RISC) but distinction is that full identity between miRNA and its target genes is not necessary. Like mRNA profile, miRNA expression varies from tissue to tissue, but is similar among individuals of a unique species.Conclusion: The main involved miRNA in lymphocyte differentiation is miR-181 that has increased in all lymphoid LINEAGEs. This Mir targets Bcl-2, CD69 and TCR-a genes. In B-cell differentiation mir-150 is the prominent decreased microRNA at Pro-B stage but mir- 17-92 cluster and mir-181 increased in pre-B and B cell stage respectively. Both miR-223 and miR-142 that are present in bone marrow increased in primary steps of T cell differentiation.