Emerging evidence has shown that microRNAs (miRNAs) can act as oncogenes in the initiation and progression of leukemia and lymphoma. Aberrant expression of oncogenic miRNAs, including miR-155, miR-17-92, miR-21, miR-125b, miR-93, miR-143-p3, miR-196b, and miR-223 promotes leukemogenesis through increasing the leukemic stem/progenitor cell population, promoting cell proliferation, blocking cell differentiation, and diminishing cell apoptosis. In addition, abnormal expression of oncogenic miRNAs, such as miR-155, miR-17-92, the miR15a/16-1 cluster, miR21, miR34a, and miR125b has been implicated in lymphomagenesis. Notably, miR- 155 and miR-17-92 profoundly changed the gene expression signatures and signal transduction pathways in various hematopoietic cells, and triggered leukemogenesis and lymphomagenesis. Therefore, miRNAs play an important role in the genesis of leukemia and lymphoma. Accordingly, oncogenic miRNAs may serve as diagnostic and prognostic factors for patients with leukemia or lymphoma, and could be used as targets for novel anti-leukemia and anti-lymphoma drug discovery.