In a field of commercial nuclear reactors, a control blade, which is inserted into and withdrawn from the core of a nuclear reactor so as to start/shut-down the operation thereof and to control the reactor power, must have soundness, high reactivity and long life. The control blade for nuclear reactors is arranged to prevent swelling of a neutron absorber so as to improve the soundness. Furthermore, even if the swelling takes place, the soundness of the control blade can be maintained. The control blade is arranged to prevent the swelling phenomenon in such a manner that boron exhibiting excellent neutron absorbing performance absorbs hydrogen which is produced as a result of reactions with the neutrons. The control blade for nuclear reactors has an upper structure member, a lower structure member, a central tie member disposed between the upper and lower structure members, a plurality of wings connected to each other by the central tie member, and neutron absorber enclosed in the wing. Each of the wings is constituted by a plate member made of hafnium metal, a hafnium alloy or an alloy the main component of which is zirconium or titanium. The neutron absorber comprises a long-lived type neutron absorber which is enclosed in accommodating holes formed in the front insertion portion of the wing which is exposed to a large amount of neutrons and a neutron absorber which is inserted into at least the major portion of the residual accommodating holes and which contains boron. A mixture of a material containing boron and a hydrogen absorber is enclosed in the accommodating holes for accommodating the neutron absorber containing boron disposed in a range from the front insertion portion, which is exposed to a large amount of neutrons, to 1/4 of the height L of an effective core portion of a nuclear reactor.