Mitochondrion, an important organelle in human body, participates in the energy metabolism. Copy number of mitochondrial DNA (mtDNA) is closely associated with diabetes, cardiovascular disease, nerve system disease and cancer etc, and mtDNA mutation often leads to multiple organ dysfunction, resulting in a series of clinical syndromes. This article reviewed the literatures concerning mtDNA copy number published in recent years, and elaborated the number, its regulative mechanism and the association with clinical diseases.