This study presents a novel synthetic approach for the preparation of cell attachable microfibers in microfluidic system. The synthesis is performed by multiphase laminar flows with spontaneous formation of carbon dioxide bubbles resulting asymmetrically porous PU microfiber. The fabricated Janus microfiber provides two distinctive properties: one is porous region to promote the cellular adhesion and the other is nonporous region rendering mechanical strength of the scaffold. The Janus microfibers show dramatic improvement of cell adhesion, proliferation, and viability over a culture period. The Janus microfiber can be used for not only alternative 2D cell culture plate but also as novel 3D scaffold for tissue engineering wihtout any need of elegant surface modification for enhancing cell adhesions.