Adhesive connections of structural components of layered composite are attractive as for manufacturing of the new aircraft or for repair of structural elements during operation. One of the most important properties of this kind of compounds is its resistance of deboning under mechanical load. Like delamination of composite laminates, it can be characterized by the fracture toughness. In this paper the effect of plasticity of an adhesive to interlaminar fracture toughness of adhesive bond of thin-walled layered composite is investigated. Experimental study of the mode I fracture toughness of adhesive bond of glass fiber/epoxy resin laminate was made using the adhesive connection with a little lower fraction of hardener of epoxy paste. As a result after 24 hours after manufacturing of the connection the signs of plasticity of adhesive was observed. There was used the ASTM standard double cantilever beam (DCB) specimen of 5 mm total thickness, 125 mm length and 25 mm width has 55 mm initial debonding. Operating forces are applied to the specimen by mean of the loading blocks. During the test with the controlled displacement of 3 mm/min rate the data "force - displacement" was digitally stored permanently with periodic stops for accurate fixing of the current size of debonding, further unloading and determination of residual extension (load points relative displacement) of specimen. Debonding continues growth during loading was observed without the unstable increment of delamination front in contrast, for example, with the high-strength laminate. The significant residual extension after unloading also was indicated. Three options of standard (ASTM D 5528-01) of processing were used: modified beam theory (MBT), compliance calibration method (CC) and modified compliance calibration method (MCC). Non-linear model of the DCB sample (V. Pavelko, K. Lapsa, and P. Pavlovskis Determination of the mode I interlaminar fracture toughness by using a nonlinear double-cantilever beam specimen. Mechanics of Composite Materials, Vol. 52, No. 3, July, 2016) was used also. In the considered range of the debonding length all methods give close results, but the obtained mode I interlaminar fracture toughness is not constant and the trend of its value stable decreasing as a function of debonding length was observed. The analytical study was focused to adequate description of mentioned phenomena that mainly connected with plastic property of adhesive. The model of debonding propagation was developed following to (G.Cherepanov. On quasi-brittle fracture, App.Math. and Mech., (G.Herrmann, Ed.), 1968, V.32, N. 6.).