The theoretical mechanical strength of the Si-O-Si bond has been evaluated by means of ab initio RHF/3-21 G* calculations on the Si2O7H6 Molecule simulating the structure of the alpha-cristobalite crystal. Deformation was applied by increasing the distance between the silicon atoms from equilibrium up to the distance of maximum force. The position of the bridging oxygen has been optimized at each step of deformation. The interatomic force, the total energy, atom charges and overlap population have been calculated as functions of the Si-Si separation. The strength of the structure appears to be equal to 1.1 mdyn, i.e the theoretical strength of the crystal for fracture along the [011] plane is equal to 64 GPa. The model used has been found to overestimate strongly the rigidity of the Si-O-Si angle and therefore the elasticity of silica. Nevertheless, the value of the ideal strength (that is determined by the strength of the Si-O bond) may be considered as a reasonable approximation.