MicroRNAs, which are small non-coding RNAs with 18-25 nt, play important roles in immune response of teleost fish. Recently, more and more differentially expressed miRNAs (DE miRNAs) associated with immune response have been identified through high-throughput sequencing and RT-PCR technology. It is assumed that if the same miRNAs are detected as DE miRNAs in several species of teleost fish challenged with different viruses/bacteria, they are likely to be evolutionarily conserved miRNAs involved in the regulation of immune gene networks in the hosts. Based on the above assumption, ten miRNAs were discovered as responding to viral challenge, and eight miRNAs were described in response to bacterial challenge. In addition, the functional study of miRNA has also attracted attention with marked progress. It has been identified that miRNAs are involved in many immune response processes, such as in preventing the virus replication, in regulating interferon response, inflammation and cell apoptosis.