Abstract Superconducting transport-properties of SmBa 2Cu 3O y (Sm123) films on MgO single crystalline substrates prepared by pulsed laser deposition were investigated. It was found that not only T c,zero values but also J c– B properties of the Sm123 films were improved by the BaZrO 3 (BZO) buffer layer on MgO substrates. T c,zero=93.8 K, J c (77 K,1 T,B∥c)=7.5 × 10 5 A/cm 2 were obtained for Sm123 films with 0.93 μm thickness. Cross-sectional TEM observations revealed that the Sm123 film grown on the BZO buffer layer has higher crystallinity compared to the Sm123 film without BZO. Thus, improvement of the microstructure for the Sm123 films led to the higher superconducting properties of the Sm123 films. In addition, the effects of the BZO buffer layer on the microstructure and correlation to the superconducting properties of the Sm123 films were explained in view of interfacial energies on the hetero-epitaxial growth.