Abstract: Embryonic stem cells are the cells that have the ability both to self-renew and to differentiate into all the mature cell types in an adult. Both of these processes are tightly regulated by genetic and epigenetic mechanisms. Increasing evi-dence indicates that a class of single-stranded non-coding RNA, known as “microRNAs”, also plays a critical role in this process. MicroRNA can bind to target mRNAs by specific base pairing, then degrade mRNAs or inhibit protein translation. Therefore, they can participate in post-transcriptiona1 regulation. Recently, scientists began to study the effect of microRNA on embryonic stem cell and found that some microRNAs are specifically expressed and form an intricate network of mi-croRNAs, regulating key transcription factors and other genes. This review focuses on the expression of microRNA in em-bryonic stem cell and their functions. We discuss some microRNA that are specifically expressed in embryonic stem cell and their regulating effect on self-renewal and differentiation.