The heat treated Nb-Mo steel with 80 kg/mm2 strength level, reported here, is a new type of steel which has been developed to replace the conventional T-1 type steel plate. The important feature of this steel is in the reduction of alloying elements in kind and amount. By such reduction, carbon equivalent (Ceq) in welding is much reduced and the improvement of weldability is achieved. It is rather surprising that this steel has sufficiently high strength and toughness as the 80 kg/mm2 level high strength steel in spite of such reduction of the alloying elements. It is found that this is caused by the coexistence of proper amounts of Nb and Mo. In this paper, the effects of Nb and Mo upon the mechanical properties and hardenability of new steel are evaluated.