An electrochemical sensor for ultratrace nitroaromatic compounds (NACs) using mesoporous SiO2 of MCM-41 as sensitive materials is reported. MCM-41 was synthesized and characterized by scanning electron microscope, transmission electron microscopy, and small-angle X-ray diffraction. Glassy carbon electrodes modified with MCM-41 show high sensitivity for cathodic voltammetric detection of NACs (including 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitrobenzene (TNB), 2,4-dinitrotoluene, and 1,3-dinitrobenzene) down to the nanomolar level. The high sensitivity is attributed to the strong adsorption of NACs by MCM-41 and large surface area of the working electrode resulting from MCM-41 modification. The voltammetric response is fast, and the detection of NACs can be finished within 14 s. SiO2 nanospheres were similarly used to modify glassy carbon electrodes for electrochemical detection of TNT and TNB. The detection limit of SiO2 nanosphere-modified electrodes is lower than that of MCM-41-modified electrodes, possibly due to the smaller surface area of SiO2 nanospheres than mesoporous MCM-41. The results show mesoporous SiO2-modified glassy carbon electrodes, particularly MCM-41-modified electrodes, open new opportunities for fast, simple, and sensitive field analysis of NACs.