A clamp structure includes a base plate, a pair of first jaws, and a pair of second jaws. The first jaws extend respectively from two sides of the base plate in a face-to-face manner and form a first jaw opening at their ends. The first jaws are also formed with a pair of cutouts that face each other. The second jaws extend respectively from the pair of cutouts towards the base plate and form a second jaw opening at their ends. Each second jaw has a hook-shaped configuration composed of a first arc near the second jaw opening and a second arc away from the base plate, wherein the first arc has a greater curvature than the second arc. A junction box and a solar panel which are bonded together can be inserted into and thus clamped by the clamp structure so as to be secured against lateral shifting.