Simplified expressions for the parameter controlling the load-point compliance and strain energy release rate were obtained for the edge crack torsion (ECT) specimen for Mode III interlaminar fracture toughness. Data reduction methods for Mode III toughness based on the present analysis are proposed. The effect of the transverse shear modulus G₂₃ on Mode III interlaminar fracture toughness characterization was evaluated. Parameters influenced by the transverse shear modulus were identified. Analytical results indicate that a higher value of G₂₃ results in a lower load point compliance and lower Mode III toughness estimation. The effect of G₂₃ on the Mode III toughness using the ECT specimen is negligible when an appropriate initial delamination length is chosen. A conservative estimation of the Mode III toughness can be obtained by assuming G₂₃ = G₁₂ for any initial delamination length.