Here, we report the structural and mechanical properties of several two-dimensional (2-D) materials by using first-principles density functional theory calculations. We find that the buckled single-layer silicene could transit to planar hexagonal silicene at a critical tensile strain of 0.20. Phonon dispersion analysis suggests that the planar hexagonal silicene under tension is stable. The Poisson's ratio of silicene and MoS2 shows strong anisotropy: it increases while stretched in the zigzag direction, but decreases when strained in the armchair direction. When stretched in the zigzag direction, the Poisson's ratio of silicene could reach 0.62.