Deformation experiments for 20(MgO or Na2O)-20Al(2)O(3)-60SiO(2) glasses were performed in simple shear geometry at 1.5-5 GPa and room temperature. An abrupt change in the thinning rate and the turning of the birefringence azimuth at a shear strain of gamma = 1-2 indicate a transition of deformation mechanism from uniaxial compression aided by densification to shear flow in the glasses. The high-dense magnesium aluminosilicate glass showed strain softening controlled by the rearrangement of the tetrahedral network. On the other hand, low-dense sodium aluminosilicate glass deformed by packing-induced flow associated with densification and via the rearrangement of the tetrahedral network at lower and higher strains, respectively. The transition of the deformation mechanism was triggered by the limitations of the densification of the tetrahedral network. The difference of deformation mechanism brought about higher strain in magnesium aluminosilicate glass than sodium aluminosilicate glass at the same stress condition. Easiness of remarkable deformation, which relaxed residual stress, and high deformability contributed to the high ductility of the MgO-aluminosilicate glass.