Cardiolipin (CL) is a unique phospholipid found in mitochondrial inner membrane. It is a key component for mitochondrial function in both respiration and apoptosis. The level of CL is an important parameter for investigating these intracellular events and is a critical indicator of a number of diseases associated with mitochondrial respiratory functions. 10-Nonyl acridine orange (NAO) is the only fluorescent dye currently available for CL detection. However, the performance of NAO is far from satisfactory in terms of selectivity and sensitivity. In this work, we report an aggregation-induced emission-active fluorogen, TTAPE-Me, for CL detection and quantification. With improved sensitivity and excellent selectivity to CL over other major mitochondrial membrane lipids, TTAPE-Me could serve as a valuable fluorescent sensor for CL quantification. The use of TTAPE-Me for the quantification of isolated mitochondria is also demonstrated.