A reciprocatable open end wrench comprises a swing member allowing to slide relative to a concave arcuate surface of a second jaw, the radius of a convex arcuate surface of the swing member is equal to that of a concave arcuate surface of the second jaw, and between the concave arcuate surface and the convex arcuate surface is provided with an engagement structure having a slot member and a peripheral protrusion so as to engage with each other. Besides, the reciprocatable open end wrench further includes a stop portion for resisting against the torque of the swing member, thereby enhancing the strength and preventing the related components to be damaged.