The architecture of mesoporous silica was successfully controlled by adjusting the concentrations of a cationic surfactant and ammonia. An excess amount of the surfactant suppressed the grain growth and then induced the formation of small grains with a diameter below 20 nm. Consequently, assembly of the small-sized grains produced a bimodal pore structure consisting of framework mesopores of 2-3 nm and textural mesopores ranging over 10-100 nm.