Spin burst tests of carbon-carbon (C/C) rings were carried out to confirm fracture criterion under high speed rotation as a function of the inner/outer diameter ratio R. The C/C rings were of a quasi-isotropic lamination type and were confirmed to have isotropic strength by static tensile tests. The burst rotation speeds of the C/C rings were found to be much lower than those expected from stress distributions. Detailed inspection of stress distributions in the C/C rings revealed that rotational unbalance inherent in C/Cs caused high average hoop stress and stress concentration at around inner radius. These effects were shown to lower fracture rotation speeds.