Both CrAlN and SiNx coatings were deposited sequentially by RF magnetron sputtering. During sputtering, thickness of SiNx layer was set to be 1 nm, while that of CrAlN layer was controlled to be 4, 20, 40, 100, and 200 nm. According to XRD results, it was revealed that grain size of the CrAlN coatings increased from 3.6 nm to 24.2 nm with the increasing thickness. From HRTEM images, the variation on grain size was attributed to the amorphous SiNx layer, which significantly retarded the continuous growth of CrAlN layer. Hardness of the CrAlN/SiNx coatings with various bilayer thicknesses was measured by nanoindentation. The relationship between grain size and hardness could be interpreted by the Hall-Petch equation, and an improved hardness around 32 GPa was achieved. (C) 2010 Elsevier B.V. All rights reserved.