Mesoporous solid acid catalyst composed of Nb-Mo oxide was prepared via a hydrothermal synthesis employing ammonium niobium oxalate and hexaammonium heptamolybdate tetrahydrate as the precursors. The assembly of fine particles with diameters of 20-34 nm was observed by field emission-scanning electron microscopy. The Nb-Mo oxide has mesopores with diameters of 1.6-2.0 nm. It was possible to tune size of the primary particles and diameter of the mesopores by changing Mo/Nb ratio. The Nb-Mo oxide exhibited high activity in Friedel-Crafts alkylation.