The Master Curve approach using miniature C(T) specimens with 4 mm thickness is promising for directly determining the reference temperature of reactor pressure vessel (RPV) steels because they can be taken from the broken halves of the Charpy specimens used in the surveillance program to monitor neutron irradiation embrittlement. The relative dimensional tolerances of standard C(T) specimens are provided in the present ASTM E1921 standard; consequently, the absolute dimensional tolerances are stricter for smaller specimens. In this study, the effect of the tolerances of key dimensions on the elastic–plastic equivalent stress intensity factor derived from the J-integral, KJ, was calculated using three-dimensional finite-element analysis. Even if the dimensional tolerances for the miniature C(T) specimens based on the present standard were mitigated in some degree (as examples, the tolerance of specimen thickness of ±0.08 mm was mitigated to ±0.1 mm; and the tolerance of specimen width of ±0.04 mm was mitigated to ±0.1 mm), the variations of KJ and the reference temperature were negligibly small. Furthermore, the use of the mitigated dimensional tolerances with adequate accuracy for evaluating the fracture toughness was ascertained.