Abstract: A straightener for straightening longitudinal cylindrical bars includes: a machine body, a driving wheel unit having plural driving wheels rotatably mounted on one side of the machine body, an idle plate assembly having plural idle plates retained on the opposite side of the machine body interlacedly facing the plural driving wheels, and a driving motor for driving the driving wheels, so that upon a transverse feeding of a deformed bar into a slit defined by the driving wheels and the idle plates and rotating the wheels as driven by the motor, the bar can be arcuately moved downwardly through the arcuate slit and rotatively bitten by the wheels and the idle plates to straighten the bar.