To explore the changes of mitochondrial membrane potential in rat cadiomyocyte apoptosis by fluorescent JC 1, H 2 O 2 was used to induce cadiomyocyte apoptosis, and JC 1 in combination with flow cyometry was used to detect the changes of mitochondrial membrane potential in the early stage of cadiomyocyte apoptosis. The results showed that living cardiomyocytes had a high mitochondrial membrane potential, JC 1 aggregates were formed in the inner membrane of mitochondria and emitted orange red fluorescence. H 2 O 2 caused the decrease of mitochondrial membrane potential, JC 1 aggregates were dissociated to monomer,which emitted green fluorenscence. So the red fluorescence decreased. It is suggested that JC 1 in combination with flow cyometry is an ideal method to detect the changes of mitochondrial membrane potential.