MicroRNAs(miRNAs)are a group of endogenous non-coding small RNA molecules of 21-25 nt. They can regulate gene expression at the transcriptional level through interaction with inhibition of target gene translation, participating in a variety of important biological processes. Increasing studies have found that miRNA plays a very important role in the development and progression of tumor, and that several miRNAs are closely related to the development of some tumors. Not only the miRNAs in the tissue, but also those in the peripheral blood are involved in the tumorigenesis, and they may be taken as biomarkers for diagnosis of patients, casting new lights on the early diagnosis in clinical practice. Detection of tumor-specific miRNAs in the blood is of great importance for the early diagnosis of tumors.