The crack growth behavior in the high crack velocity region (region III) for binary sodium silicate glasses was investigated by using a DCDC (double cleavage drilled compression) specimen under compressive stress. For 15mol% Na2O glass, despite the anomalous characteristic of the bulk modulus, subcritical crack growth in region III was detected. The slope of stress intensity factor (KI)-crack velocity (v) curve for silicate glass depended on the number of non-bridging oxygen per SiO4 tetrahedron rather than on the alkali content in the glass. This compositional dependence of the slope of KI-v curve for silicate glasses could not be explained in terms of thermally activated crack growth process where the crack-tip shape would vary with the pressure derivative of bulk modulus. The smaller KI-v slope and the higher KIC value of 35mol% Na2O glass may be related to the structural change from a three-dimensional network to a two-dimensional network because of the introduction of many non-bridging oxygens into the silica network.