Hollow core polyacrylonitrile (PAN) microfibers with nano- and microporosity are synthesized based on the regeneration of PAN from dimethyl sulfoxide in a hydrodynamic flow-focusing microfluidic device. This technique is a fast and one-step approach to fabricate such core-shell microfibers. The fiber size (20-45 mu m) and the nanopore size (90-300 nm) are well controllable by simply changing core-sheath flow rate and PAN concentration.