Dependence of superconducting T c upon Pr-substitution concentration x was studied systematically inR 1 - x Pr x Ba 2 Cu 3 O 7 - δ system with R = Nd, Dy, Tm, Yb and Lu. It was found that there is a T c -plateau in the range of low x as R 3 + radius r i =< r i (Dy). The Pr-concentration width corresponded by T c -plateau exhibits a R 3 + ion size effect. It is suggested that the ion size effect of T c -plateau might result from the change of local state of Pr 4f electron. A criticalR 3 + radius, r i c , is suggested and superconductivity disappears as r i > r i c .