A flexible circuit board includes two copper clad laminates, a circuit pattern and two bonding layers. Each copper clad laminate includes an insulating base and an outer circuit layer. The circuit pattern is located between the two copper clad laminates. The circuit pattern includes a linear signal line, two grounding lines located at two opposite sides of the linear signal line, and two hollow areas located between the linear signal line and the grounding lines. Each bonding layers is located between the circuit pattern and a corresponding copper clad laminate. Each boding layer defines a slot without adhesive therein. The bonding layers are spaced from the linear signal line by the slots. The slots and the hollow areas cooperatively define an air medium layer enclosing the linear signal line. A method for manufacturing the flexible circuit board is also provided.