MicroRNA(miRNA) are endogenous ～22 nt non- coding small RNAs broadly expressed in plants and mammalians. MiRNAs are involved in the negative regulation of gene expression through the targeting of mRNAs during the process and activity of life such as cell proliferation and differentiation, apoptosis as well as the development of diseases. Recently, numerous studies have demonstrated that the expression of miRNA is of time and tissue specificity. It has been predicted that each miRNA may target more than one or hundreds of mRNAs. In addition, the functional targets of miRNAs may be different in different cells or the different states of the same cell. All these results suggested that miRNA and its functional targets may be dynamically variable with the time and space. MiRNAs may play complex roles in human body. In this paper the dynamic change of miRNA and its targets has been reviewed and it may provide new idea for the study of miRNAs.