A Differential line inductor is proposed for a differential power amplifier. The proposed differential line inductor is composed of two conventional line inductors rearranged to make the current direction of the two line inductors identical. The proposed line inductor is simulated with a 2.5-D and a 3-D EM simulator to verify its feasibility with the substrate information in a 0.18-${mu}m$ RF CMOS process. The inductances of various line inductors implemented with printed circuit boards were measured. The feasibility of the proposed line inductor was successfully demonstrated.