MicroRNAs (miRNAs) play important roles in cell division, development, maintenance of normal physiological function, occurrence, development and outcome of the diseases in metazoan animals, plants. The miRNA sequences stored in miRBase are usually taken as the only known miRNA sequences. In fact, people often ignore the varieties of the miRNA sequences and underestimate the diversities and importance of the functions of miRNA expressed in non-primary forms. Due to the rapid development and a wideranged application of sequencing technology, the variations in sequences and expression levels of mature miRNAs could be easily detected from the data obtained by the use of second-generation sequencing technology. This review intends to summarize the progress on 5′ -end, 3′ -end, or internal variants of miRNAs, the expression differences of miRNA variants in different developmental stages and different tissues, and dominant arm switch of miRNAs in recent years.