The elastic properties of cristobalite and quartz, the typical polymorphs of SiO 2 , are studied with particular emphasis on the structural phase transition and the high-temperature phase. Using the equilibrium molecular dynamics (MD) method with a stress-fluctuation formula, we have successfully evaluated the adiabatic elastic constants ( ij C ) of both cristobalite and quartz in a wide temperature range, including the a -b phase-transition region. In the results for cristobalite, the anomalous property of a negative Poisson's ratio appears over the entire temperature range of 300 -1800 K, where the bulk modulus is extremely low compared with the shear modulus. However, the mechanisms differ between the a -and bphases, according to the structural data obtained from the MD simulations. On the other hand, quartz exhibits a negative Poisson's ratio in the narrow temperature region between ca. 750 and 825 K before the phase transition occurs. After the transition to the b -phase, the bulk modulus is shown to increase sharply, and a positive value of Poisson's ratio is recovered. For cristobalite as well as quartz, we have confirmed that the recovery of bulk ij C 's in the b -phase can be attributed to internal relaxations, which arise from the cooperative motions of corner-linked SiO 4 tetrahedra.