The variations of superconductivity in Gd 1 - x Pr x Ba 2 - y Sr y Cu 3 O 7 - δ have been studied. The system keeps @'123@' structure through the solid solution ranges of 0 =< x =< 1 and 0 =< y =< 1. The critical Pr concentration for the quenching of superconductivity is about 0.45 when y = 0, and is increased to about 0.6 when y = 1.0. In the series Gd 0 . 5 5 Pr 0 . 4 5 Ba 2 - y Sr y Cu 3 O 7 - δ , it is observed that superconductivity reappears when y >= 0.5. These results further indicate that there is an @'ion-size effect@' on T c at the Ba-site, which was discussed in terms of the Pr-O covalent bonding effect.