Though interface has been known for a critical role in determining the properties of conventional composites, its role in polymer nanocomposites is still fragmented and in its infancy. This study synthesized a series of epoxy/clay nanocomposites with different interface strength by using three types of modifiers: ethanolamine (denoted ETH), Jeffamine (R) M2070 (M27) and Jeffamine (R) XTJ502 (XTJ). XTJ created a strong interface between clay layers and matrix because it bridged the layers with matrix by a chemical reaction as proved by Fourier transform infrared spectroscopy; M27 produced an intermediate interface strength due to the molecular entanglement between grafted M27 chains and matrix molecules; the interface made by ETH was weak because neither chemical bridging nor molecular entanglement was involved. The studies of mechanical and thermal properties and morphology at a wide range of magnification show that the strong interface promoted the highest level of exfoliation and dispersion of clay layers, and achieved the most increment in Young's modulus, fracture toughness and glass transition temperature (T-g) of matrix. With similar to 1.3 wt% clay, the critical strain energy release rate G(1c) of neat epoxy improved from 179.0 to 384.7 J/m, 115% improvement and T-g enhanced from 93.7 to 99.7 degrees C, 6.4% improvement. (C) 2010 Elsevier Ltd. All rights reserved.