A knee brace for rehabilitation provides a stable support for an injured knee using a four-crank mechanism. Positioning pins are used to fix limiting blocks pivoted to the upper and lower braces. It limits the moving range of the crank between the upper and lower braces, restricting the moving angles of the upper and lower braces. Besides, the buffer pads are covered at the pivotal axes of the upper and lower braces in order to alleviate the collision between joint surfaces during the motion of injured knee. Such a mechanism protects the rehabilitating knee joint. The disclosed structure can solve the failure problem of the knee brace for rehabilitation due to a loose pin in the prior art. The counterforce to injured knee from the performance of knee brace reduced for rehabilitation can prevent the user's knee joint from further injuries.