Abstract: Colorectal cancer (CRC) is one of the most common digestive system neoplasms in China. Recently, the morbidity and mortality rates of CRC continue to increase with the improvement of people's living standard and the change of the dietary structure. It was accepted that the etiology of colorectal cancer is a multi-factor and multi-step process. However, the mechanism of CRC is still unclear. MicroRNAs (miRNAs) are a class of -21 nucleotide non-coding RNAs and function as a negative regulator of gene expression after transcription, participating in tumor angiogenesis, tumor cell proliferation and invasion. MicroRNA-21 (miR-21), as an important oncogenic RNA, plays a vital role in the development and progression of CRC. This review summarizes the advances in research on miR-21 in colorectal cancer.