Helical carbon nanofibers (HCNFs), such as the carbon nanocoil (CNC) and the carbon nanotwist (CNTw), were synthesized by catalytic chemical vapor deposition using a substrate. HCNFs are classified into round and angular types according to the fiber cross-sections. When four types of HCNFs (round-type CNC and CNTw, angular-type CNC and CNTw) were acidified in a 30% hydrogen peroxide solution, only the angular type CNC was found to show a drastic shape change. The shape change observed was a splitting followed by a flattening of the angular type CNCs. The CNC was split into two or three thinner flat fibers. As a function of the treatment temperature, the weight of the CNCs decreased above 80 degrees C and the CNCs were etched effectively at 140 degrees C. The longer the reaction time the lower the weight of the CNCs, and the weight loss reached a saturation point when the reaction time was greater than 45 min at 140 degrees C. The splitting and flattening of the CNC occurred during the weight loss process. To analyze the CNC structure, electron tomography of the as-grown and acid-treated CNCs was obtained using a computerized tomograph system with transmission electron microscopy (TEM). The 3D-images were constructed using the TEM images collected at different tilt angles. The 3D image reconstructions showed that the CNCs had a tubular structure and were composed of several helical fibers which act as frames.