The influence of RE (RE = Yb, Sm) substitutions on the microstructure and superconducting properties of Y-Ba-Cu-O (YBCO) bulk superconductors has been studied for different RE concentrations and their combinations. Precursor powders YBa 2 Cu 3 O 7-δ (Y-123), Y 2 O 3 and CeO 2 were enriched with different amounts of Yb2O3 and (or) Sm2O3 powders with the aim to increase critical current density, i.e., Jc, in self-field as well as in higher magnetic fields, by introducing additional pinning centers. YBCO bulk superconductors with RE 2 O 3 addition were prepared by the optimized top-seeded melt-growth process in the form of single grains. Microstructure analysis revealed that RE 2 O 3 addition leads to a higher amount of smaller Y 2 BaCuO 5 (Y-211) particles, which is related to high critical current density (J c i∼ 7 × 10 4 A/cm 2 ) of the samples with RE 2 O 3 addition in low magnetic fields. In this paper, the effect of RE 2 O 3 addition on Y-211 particle size, critical temperature Tc, and critical current density Jc is reported. © 2015 IEEE.