A simple and reliable approach for fabricating circular nanofluidic channels in polydimethylsiloxane (PDMS) microfluidic systems is described, which uses core-sheath nanofibers as sacrificial molds. The core-sheath structures consist of an electrospun polyvinylpyrrolidone (PVP) core and a sputtered aluminum sheath. The rupture of the sheath during master template releasing allows easy removal of the nanofibers to form the nanochannels. Straight nanochannels with the diameter as small as 390 nm are demonstrated. This technology is advantageous over existing nanochannel fabrication approaches in reduced risks of fluidic leakage and channel blocking, simpler fabrication process, lower cost and easier dimension control. This work provides a solid technical basis that enables development of various on-chip analytical devices for investigation of the unique transport phenomena at nanoscale.