We used first-principles methods to calculate the elastic properties of rutile (R) structure and monoclinic (M 1 : space group P21/c, M 2 : space group C2/m) structure VO 2 , including single-crystal elastic constants c ij 's, polycrystalline bulk modulus, shear modulus, Young's modulus and elastic anisotropy ratio. We found that the energy difference among the R, M 1 and M 2 phases is small, indicating that it is easy to transit among them under a perturbation. Furthermore, from the pressure dependence of c ij 's, we also found that the structural instability (or phase transition) will occur when the volumes of the three phases are slightly smaller than their equilibrium volumes. Additionally, the R and M 2 phases are predicted to be harder than the M 1 phase, indicated by their larger bulk moduli and shear moduli. The elastic anisotropy of the M 2 phase is larger than the M 1 and R phases. The presently predicted elastic properties of VO 2 provide helpful guidance for the strain energy estimation and stress analysis in nano-electronic devices.