Colorectal cancer (CRC) is the third most common malignant tumor worldwide, affecting over 1.23 million individuals annually. Over the past several years, the role of microRNAs (miRNAs) in CRC cells have been extensively investigated. Recently, it was discovered that extracellular miRNAs circulate in the blood of healthy individuals as well as CRC patients. Serum circulating miRNAs as novel biomarkers may be a new approach to CRC diagnosis. However, their secretory mechanism and biological function have not been fully elucidated. In this review, the main serum miRNAs used as potential diagnostic biomarkers for CRC, such as miR-92, miR-141, miR-29a, miR-21, miR-221, miR-601 and miR-760, were summarized. In addition, the mechanism of miRNA secretion from CRC cells was discussed. It is recommended that additional studies focus on the usefulness of serum miRNAs for early CRC diagnosis, prognosis and therapeutics.