A flexible circuit board uses a specific structure to alleviate mechanical stress thereof. The flexible circuit board has a flexible film, a plurality of inner leads, a plurality of outer leads, and a plurality of connection portion. Each of the connection portions a corresponding one of the inner leads with a corresponding one of the outer leads. A first width of the inner leads is greater than a second width of the outer leads. Due to rounded concave sections and rounded convex sections of the connection portions, if the flexible circuit board is bent, the mechanical stress around corners of joint portions of the connection portions with the inner leads and the outer leads could be alleviated.