Single fiber fragmentation tests were carried out to measure the properties of the fiber-matrix interface in several carbon fiber (CFs)/epoxy composite systems. Four kinds of CFs were studied: (1) primary CFs (used as received); (2) desized CFs (sizing removed through thermal treatment); (3) resized CFs (deposited with epoxy sizing by solution); (4) carbon nanotube (CNT)-grafted CFs (grown with CNTs using a chemical vapour deposition method). The interfacial shear strength decreased by around 30% for the desized CFs and the CNT-grafted CFs compared with the pristine CFs. The value of interfacial shear strength for the resized CFs was 20% larger than that of the desized CFs. There is a good agreement between the results of single fiber fragmentation tests and that of contact angle tests.