The thickness of iron-based accident-tolerant fuel (ATF) cladding is discussed in this technical note because its thickness tends to be reduced from conventional zirconium alloy cladding. Structural stability may be lost if the thickness cannot withstand external pressure. Thus, the minimum allowable thickness of the ATF cladding is studied here from the viewpoint of preventing a cladding collapse. The elastic buckling theory is used to obtain the minimum thickness. The uncertainties of the mechanical properties and dimension tolerances are taken into consideration. The ovality of the cladding is also incorporated. An example calculation is carried out for APMT cladding. It is evaluated that the minimum thickness is 0.45 mm when the safety factor against the buckling is set as 2.0 and 1% of the cladding radius is accommodated for the ovality. A reference guideline of the minimum thickness depending on the mechanical property variation is suggested.