A Y2BaCuO5 buffer (Y211) block, as a diffusion barrier for samarium atoms of a top seeded melt growth (TSMG) processed YBa2Cu3O7 − y (Y123) superconductor using a SmBa2Cu3O7 − d (Sm123) seed, was examined in terms of the buffer thickness (t). Disc-type Y211 blocks with various t were inserted at the seed/compact interface and subjected to melt growth heating cycles. The superconducting transition temperature (Tc) and critical current density (Jc) were measured for the three different top surface regions: (1) the region just below the seeded Y211 buffer block, (2) the intermediate region between the buffer block and the sample edge, and (3) the sample edge. The Tc values of regions 1 and 2 of the samples without the buffer and with thin buffers were lower than that of region 3, and the superconducting transition widths were larger. As t was increased to 3 mm, the susceptibility curves of the three regions showed the same Tc of 90 K and sharp transitions. The Jc–B curves of regions 1 and 2 without a buffer and with thin buffers showed the peak effect due to the formation of Y1 − xSmxBa2Cu3Oz. The peak effect almost disappeared when t was increased to 3 mm. Y211 particles and the Y123 matrix in the buffer block acted as an effective samarium absorber by forming mixed rare-earth 211 particles and Y1 − xSmxBa2Cu3Oy. These results suggest that the Y211 buffer block technique can be applied to the fabrication of large single grain Y123 superconductors with a uniform Tc and Jc distribution on the top surfaces.