Highly ordered mesoporous silica nanowire (NW) arrays were fabricated inside the pores of porous anodic aluminum oxide (AAO) templates using a simple sol-gel method at different aging conditions. High-resolution transmission electron microscopy investigation shows that the structure of silica NWs is sensitive to the template structure and aging environment. For the samples aged with the presence of water, all the nanochannels in the silica NWs were found to be circular around the NW axes. However, for those samples aged without water, most silica NWs have a tube-like structure due to the confinement effect of the AAO pore-walls. Those tube-like silica NWs formed in large AAO pores generally consist of a core (hexagonally arranged nanochannels parallel to the NW axis) wrapped by shells. The formation mechanism for these novel structures of mesoporous silica NWs were proposed and discussed. (c) 2005 Elsevier B.V. All rights reserved.