We report the fabrication of highly densified mesoporous bulk silica consisting of colloidal mesoporous silica nanoparticles (MSNs) by using a hydrothermal hot-pressing method. No cracks or voids are found in the synthesized bulk. The mesoporous structure of the bulk is maintained as well as that of the original MSNs. The MSN-based bulk with large pore volume shows potential as a low-k material.