The outcomes of standard test of a carbon/epoxy layered composite using DCB specimen for measuring of the I-mode interlaminar fracture toughness (IFT) were analysed. Using the test results and earlier developed the model of quasi-static growth of crack (delamination) in layered composite, the investigation of variable properties of IFT was continued. It is shown that the use of stationary continuous functions for mathematical description of IFT does not allow to completely satisfy to the data of test. The simplest type of stationary discontinuous function is selected for analysis and simulation: the periodic piececonstant function. Using the model of quasi-static growth of crack and the mentioned type of variable IFT function the process of a crack propagation in DCB sample at quasi-static loading with control of displacement is simulated. Comparison of IFT function and the crack resistance (R-curve) together with other outcomes of simulation are the base of proposed procedure of variable IFT estimation from DCB Standard test.