Mode III interlaminar fracture toughness in CFRP is evaluated experimentally by•• using the Edge Crack Torsion (ECT) test. A material system used is T700S/2500 carbon/epoxy system. Laminate configuration is [90/(+45/-45)3/(-45/+45)3/90]s with a delamination introduced by a 25μm-thick kapton film at the mid-plane along one edge. The specimen size is 90mm long, 40mm wide and 4.2mm thick. The film lengths were 0, 5, 10 and 15 mm. A natural crack is produced with a wedge from the film edge. The resulting crack lengths are from 8mm to 18mm. Two data reduction methods are used to calculate mode III interlaminar fracture toughness GIIIC at the maximum load and load at onset of nonlinearity. Soft X-ray radiograph is used to observe delamination propagation. Three-dimensional finite element analysis is performed to calculate the distribution of mode I, mode II, and mode III energy release rate along the delamination front. The validity of the experimental method is also discussed.