We report a new approach to fabricate nanochannels on silicon-on-insulator (SOI) wafers using conventional micromachining techniques. Proper selection of the size of the photomask-window and the thickness of the top silicon layer is necessary to obtain nano-sized regions. Silicon anisotropic wet etching followed by an additional reactive-ion-etching (RIE) process and a second silicon wet etching step resulted in long channels (1 cm) of about 200 nm width and 100 nm depth. Finally, we demonstrated the ability of the nanochannels to stretch random coiled DNA by applying YOYO-1 stained lamda-DNA to the nanochannel sealed by PDMS polymer using fluorescence microscopy. This fabrication method provides a basis for simple and cost-effective mass production of nanochannels with controllable dimensions. It is therefore expected that the nanochannels fabricated have great potential for biological applications.