Assembly of mesostructured silica using Pluronic P123 triblock copolymer (EO(20)-PO(70)-EO(20)) and n-butanol mixture is a facile synthesis route to the MCM-48-like ordered large mesoporous silicas with the cubic Iad mesostructure. The cubic phase domain is remarkably extended by controlling the amounts of butanol and silica source correspondingly. The extended phase domain allows synthesis of the mesoporous silicas with various structural characteristics. Characterization by powder X-ray diffraction, nitrogen physisorption, scanning electron microscopy, and transmission electron microscopy reveals that the cubic Iad materials possess high specific surface areas, high pore volumes, and readily tunable pore diameters in narrow distribution of sizes ranging from 4 to 12 nm. Moreover, generation of complementary pores between the two chiral channels in the gyroid Iad structure can be controlled systematically depending on synthesis conditions. Carbon replicas, using sucrose as the carbon precursor, are obtained with either the same Iad structure or I4(1)/a (or lower symmetry), depending on the controlled synthesis conditions for silica. Thus, the present discovery of the extended phase domain leads to facile synthesis of the cubic Iad silica with precise structure control, offering vast prospects for future applications of large-pore silica materials with three-dimensional pore interconnectivity.