microRNAs (miRNAs) are a class of non-coding small RNAs with the length of about 22 nucleotides. More than 35 000 miRNAs have been found in plants, animals and viruses since its discovery in Caenorhabditis elegans in 1993. They participate in a variety of biological processes involved in cell differentiation, apoptosis, metabolism, signal transduction, immune responses, and act as critical transcription factors. miRNAs can be encoded both by viruses and host cells. Virus-encoded miRNAs can alter the internal environment of host, and host-encoded miRNAs also have effect on survival of virus. This article aims to review the functions of miRNAs in virus-host interactions.