<p>Droplets are widely used in the fields of chemical and biological research, such as drug delivery, material synthesis, point-of-care diagnostics, and digital PCR. Droplet-based microfluidics has many advantages, such as small reagent consumption, fast reaction time, and independent control of each droplet. Therefore, various micro-droplet generation methods have been proposed, including T-junction breakup, capillary flow-focusing, planar flow-focusing, step emulsification (SE), and high-aspect-ratio (HAR) channel confinement. In this study, we propose a novel design of microfluidic device for simultaneous generation of monodisperse droplets with two different sizes using the HAR microchannel array on a single device. The fabrication method was already developed in previous studies, based on basic MEMS processes including photolithography, anisotropic wet etching of Si, polydimethylsiloxane (PDMS) molding and plasma bonding between Si channel and PDMS mold. The proposed microfluidic device could be operated in three different modes, depending on the oil flow direction. That is, it could generate two monodisperse droplets with different sizes simultaneously or one monodisperse droplet by choosing one inlet among three inlets. Moreover, all droplets for three different modes were successfully generated for a wider range of flow rates of oil (continuous phase) and sample (disperse phases) regardless of the flow fluctuations. The asymmetric channel structures affected the size and uniformity of droplets, depending on the oil flow direction. It is expected that generating various size of monodisperse droplets without the efforts for splitting as well as the control of flow rates may be used in pharmaceutical and food industrial field where parallel experiments based on various sized droplets are needed.</p>