Accurate calculation of the mechanical properties of the coatings needs finding out the sources of errors. One of the important sources of errors is the presence of pre-existing defect along the interface. In this study we consider elastic-plastic coating on elastic-plastic substrate system and the cohesive zone model embedded in the finite element code is used to simulate the interface between coating and its substrate. The aim of this paper is to investigate the effect of interfacial crack growth in the presence of a defect on deformation behavior and some mechanical properties such as hardness and modulus of elasticity in Cu/Si coating system. The results reveal that when the position of the defect is nearer to the shear stress concentration point, the crack will initiate sooner. Crack propagation results in some errors in calculation of the hardness and modulus of elasticity and according to the range in which the interfacial crack has grown, the amount of errors is different. The position of the pre-existing defect, even before any crack initiation, also has substantial effect on the distribution of shear stress along the interface during loading. Numerical study shows that the variance of the nanoindentation test results will significantly increase in specific range of indentation depth. (C) 2013 Elsevier B.V. All rights reserved.