A heat treating process for a rod by the use of a fluidized bed consisting of alumina, silicon sand, zircon sand, and the like, is disclosed for adjusting the strength of the rod due to its cooling within the fluidized bed. While prior art heat treatment processes of this type adopt an attempt to vary the temperature of the fluidized bed for varying the cooling rate of the rod, such results in the need to use costly heat-resisting apparatus for the fluidized bed and equipment associated therewith. In accordance with the heat treatment process of the present invention, a rod is introduced through means of an inlet opening into the fluidized bed, and subsequently, the rod passes through the fluidized bed in an upwardly inclined manner so as to exit from the surface of the bed. The fluidized bed in this case has a predetermined length and the rod is fed at a predetermined speed. Thus, the varying soaking times of the rod within the fluidized bed are obtained by varying the distance from the inlet to the point where the rod emerges from the surface of the fluidized bed, so as to thereby adjust the resulting strength of the rod. This process has no need to vary the temperature of the fluidized bed and yet permits the adjustment of the strength of the rod to be achieved over a wide range, with simple apparatus and operations, while retaining existing productivity.