Molecular dynamics (MD) simulation is performed to study the structural evolution of single-layer MoS2 nanosheets under tensile loading and a phase transition from hexagonal structure to quadrilateral one is observed at a large strain when loaded along the zigzag direction but not along the armchair direction. Density functional theory (DFT) calculation illustrates that the newly generated quadrilateral phase is metallic. Further loading along the zigzag direction promotes an inhomogeneous distribution of lateral and shear stress around the phase boundaries due to local mechanical mismatch. As a result, periodic rippling parallel to the loading direction emerges and the wavelength and wave height change with strain according to lambda similar to epsilon(-1/4) and h similar to epsilon(1/4). Accordingly, a graded distribution of strain can be produced or modulated by a simple tensile loading and the strategy might be utilized to enhance the photoelectrical properties of 2D materials and design strain-tunable nanodevices. (C) 2017 Elsevier B.V. All rights reserved.