We propose a method for investigating the field-effect doping and the capacitance in nano-scale systems by first-principles calculations. Utilizing this method, we demonstrated the field-effect doping into the water bi-layer (channel) sandwiched by a pair of gate electrodes. We found that the H₂O molecules rotated around the O atoms artificially pinned, accompanied by the bias application between the channel and gate electrodes. We confirmed that Q-μ relation (capacitance) was also modified through this rotation of the molecule. We believe that this kind of bias-induced structural modifications of the channel materials can be utilized for designing nano-electronic devices with a variety of novel and functional characteristics. [DOI: 10.1380/ejssnt.2005.453]