A method is presented for determining by numerical integration a diffusion coefficient, Dz, applicable to the axial direction of a control rod follower region in a sodium-cooled fast reactor. By comparing criticality values from transport and diffusion theory for a simplified reactor model, we are able to show that this Dz applies well to the followers from the viewpoint of the calculated criticality factor, keff. By use of the same model, an inter comparison is also made between the present and other definitions of Dz. By using the present Dz, as a conclusion, we show that the currently used D (= 1/3Σ) for control rod followers leads to an underestimation of keff by ∼0.3% for a typical 300-MW(electric) class liquid-metal fast breeder reactor with one-third of the control rods inserted, which may increase to 0.4 to 0.5% for a case where all control rods are withdrawn.