Mitochondria are organelles that have important functions in oxidative phosphorylation, fatty acid oxidation and apoptosis signaling. They have two distinct membranes, outer membrane (OM) and inner membrane (IM). IM contains respiratory chain complexes that produce ATP. IM is rich in cardiolipin, a specific phospholipid reportedly having a critical role for organizing super-complex formation of respiratory chain complexes. IM abundant in cardiolipin exhibits resistance to extraction by digitonin (a non-ionic detergent), whereas the detergent easily lyses OM. Therefore, digitonin is useful to separate mitoplast (IM plus matrix) and OM from mitochondria. Here, we describe a method to isolate mitochondria from HeLa cells, and a method to isolate mitochondrial outer membrane proteins and inner membrane proteins by using digitonin. This method is applicable also to other types of cultured cells such as COS-7.