A utilization of a miniature compact tension (Mini-C(T)) specimen is expected to enable effective use of limited remaining surveillance specimens for the structural integrity assessment of a Reactor Pressure Vessel (RPV). For developing a direct fracture toughness evaluation method using Mini-C(T) specimen in the upper-shelf temperature range as well as ductile-brittle transition temperature range, this study is aimed to experimentally characterize the Mini-C(T) specimen’s size effect on ductile crack growth resistance and interpolate its mechanism. Mini-C(T) specimen and 0.5T-C(T) specimen were prepared from a Japanese RPV steel SQV2A, and the ductile crack growth tests were conducted on them at room temperature. As a result, the crack growth resistance of Mini-C(T) and 0.5T-C(T) specimens are comparable if the crack extension Δa is less than 0.5 mm. On the other hand, if Δa exceeds 0.5 mm, the crack growth resistance of Mini-C(T) specimen becomes lower than that of 0.5T-C(T) specimen. The measurements of stretch zone width and depth support the fact that the fracture toughness for ductile crack initiation of Mini-C(T) specimen is lower than that of 0.5T-C(T) specimen. From the rotational (crack mouth opening) deformation of Mini-C(T) specimen was measured by simultaneously measuring load-line and front face displacements. The distance between the crack tip and the rotation center of Mini-C(T) specimen is smaller than that of 0.5T-C(T) specimen during the test. Furthermore, The plastic zone in front of the crack tip reaches the rotation center up to the crack extension of Δa = 0.3 mm on Mini-C(T) specimen, indicating that the mechanism of the specimen size effect of Mini-C(T) specimen is likely a plastic constraint due to the influence of the rotation center locating near the crack tip. This suggests that the specimen size effect of Mini-C(T) specimen on ductile crack growth resistance is expected to be corrected by considering an effect of the plastic constraint.