Recently, more studies have been conducted in chemical and biological applications using microfluidic or nanofluidic devices.1 Polymer-based materials have been newly developed in this field due to the great needs of easy processing, cost-effectiveness and clarity for the material. However, it is still challenging to control of the surface properties of these devices on demand. Especially, for biological analysis or detection, micro-fluidics should handle aqueous samples but, most of the current materials in use for micro-fluidic devices are relatively hydrophobic (such as PDMS, PMMA and cyclo-olefin-co polymer, etc). Therefore, they usually need an extra assistance rather than a capillary force to flow the aqueous samples. In this paper, we utilized layer-by-layer deposition of polymer to modify the surface of the micro-channel of the device in order to control surface properties of the micro-channel. We have been studied polyelectrolyte multilayer(PEM) coatings to control surface wettability of the open structures and found various hydrophilic films. Here we demonstrate polyelectrolyte multilayer film as an effective coating for inner surface of micro-fluidic devices to lowering the water contact angle, so that the aqueous fluid will travel smoothly with the channels. Compared to the other surface treatment method such as base cleaning or plasma irradiation, the PEM coating exhibit highly sustained water wettability. Polyelectrolytes used for this study are weak polyelectrolytes including biodegradable polymer such as poly(hyaluronic acid) (HA) for future biological applications.