Using a pure metal interlayer to relieve residual stress and enhance adhesion of hard coatings on substrate materials has been commonly adopted in industry and extensively studied in last two decades. However, due to the difficulty in accurately measuring residual stress in individual layer of a bilayer coating, the effect of stress relief by a pure metal interlayer has not been fully understood. Recently we proposed a method combining average X-ray strain (AXS) method and nanoindentation (NI), by which the residual stress of hard coatings can be accurately measured with an uncertainty < 10%, which provided an effective tool to measure the layer stress. Since TiN/Ti is one of the most popular bilayer combinations, TiN/Ti coating was chosen as the model system in this study. The objective of this study was to accurately measure the residual stress in the individual layer of the TiN/Ti bilayer specimens using AXS combined with NI methods; from the results, a theoretical model was developed to further explore the effect of Ti interlayer thickness on relieving residual stress of the TiN/Ti bilayer specimens. TiN specimens with three different Ti interlayer thicknesses, 50, 100 and 150 nm, were deposited on Si substrate using unbalanced magnetron sputtering. The residual stresses in TiN top layer and Ti interlayer were separately determined by combining NI and AXS method using both lab X-ray and synchrotron Xray sources, and the overall stress of the entire TiN/Ti specimen was measured by laser curvature technique. The results showed that the Ti interlayer with thickness larger than 100 nm could relieve residual stress of the bilayer specimen. However, when the interlayer thickness was insufficient (50 nm), the stress of the entire specimen may increase instead of decrease even the interlayer was added. It was found that the Ti interlayer with thickness of 50 nm was subjected to a compressive stress, while interlayers with thickness of 100 nm and above were under a tensile stress. A physical model was proposed to describe the stress variation with the interlayer thickness, which could delineate the experimental findings where a switch of stress state from tension to compression occurred in the interlayer at a critical interlayer thickness. (C) 2017 Elsevier B.V. All rights reserved.