A method for determining antenna rule for a radio-frequency (RF) device includes the steps of forming a gate structure on a substrate, forming a source/drain region adjacent to the gate structure, forming a first metal routing on the source/drain region, and then forming a second metal routing on the gate structure. Preferably, a sum of an area of the first metal routing and an area of the second metal routing divided by an area of the gate structure is less than a ratio.