A microfluidic channel system to generate Janus droplets is designed and fabricated, where the term Janus droplet refers to a chemically biphasic droplet. It is demonstrated that Janus droplets are formed from elongational breakup of coflowing core fluids which are constrained by a sheath fluid on both sides of them. Rhodamine B is adopted as an indicator to indentify generated Janus droplets. Monodisperse Janus droplets have been generated in a controllable manner such that those with average diameters of $26{pm}1.24{mu}m$, $31{pm}1.44{mu}m$ and $34{pm}2.28{mu}m$ are formed in accordance with flow rate ratios between the sheath fluid and the core fluids, 30,7, 36.4 and 44.4, respectively. Generation of Janus droplets, demonstrated in the present study, has seen a new application in the areas of biotechnology and bioengineering, where enhanced mixing inside the micro bubbles can be utilized without the aid of other means of droplet generation and merging.