Dependence of superconducting Tc upon Pr-substitution concentration x was studied systematically in Ｒ1-xＰｒxＢａ2Ｃｕ3Ｏ7-δ system with R=Nd, Dy, Tm, Yb and Lu. It was found that there is a Tc-plateau in the range of low x as R３＋ radius ｒi≤ｒi(Dy). The Pr-concentration width corresponded by Tc-plateau exhibits a R３＋ ion size effect. We suggest that the ion size effect of Tc-plateau might result from the change of local state of Pr 4f electron. A critical R３＋ radius, ｒic, is suggested and superconductivity disappears as ｒi＞ｒic.