An impact beam for car doors is reinforced at its central portion. In an embodiment, the impact beam consists of a longitudinal beam stepped at two positions to allow its central portion to be thicker than its opposite end portions. In another embodiment, the impact beam consists of a longitudinal base beam body, and a hollow reinforcing beam body having a length shorter than that of the base beam body and a sectional size larger than that of the base beam body, and fitted over a central portion of the base beam body. The impact beam further comprises a beam bracketing structure for fastening the opposite ends of the impact beam on a door panel. This beam bracketing structure consists of two support brackets fixedly mounted to the door panel, and a locking mechanism for locking each end of the beam to an associated one of the two support brackets.