A method for fabricating an inductor device includes the steps of forming a plurality of trenches in a substrate by selectively etching the substrate, implanting dopants into sidewalls and bottom portion of each trench, forming an oxide layer by oxidizing the trenches and the substrate and simultaneously forming a doped layer in the surroundings of the trenches by diffusing the dopants into the substrate, and forming a dielectric layer on a resultant structure to fill the entrance of the trenches, thereby forming air-gap layers inside the trenches, thereby reducing a parasitic capacitance and a magnetic coupling.