In order to interpret the low-H peak and the particular form of M(H) curves of pure YBa 2 Cu 3 O 7−δ (YBCO) nanoparticles, many pellets of YBa 2 Cu 3 O 7−δ /Y 3 Fe 5 O 12 (YBCO/YIG) with different proportions of added YIG were prepared by pressing nanoparticles into disks of 5mm diameter and 0.5mm thickness and high temperature sintering. The crystal structure and phase composition of YBa 2 Cu 3 O 7−δ and Y 3 Fe 5 O 12 were characterized by X-ray diffraction (XRD) with CoK α radiation. Magnetization measurements vs. temperature T and applied field H were performed with a conventional magnetometer in the temperature range 4.2–110K and in magnetic fields up to 10T. The field was applied parallel to the pellet axis. Our results reveal that the application of a strong field leads to a phase transition: for H less than a threshold field H ⁎ , the superconducting nano-domains are prevalent in the sample. When the field increases beyond H ⁎ , superconducting nano-domains spins line up along the applied field direction to create a ferromagnetic phase. Phase diagrams are proposed. Field dependence of the critical current J C in YBCO pellet has revealed the existence of two pinning mechanisms: a pinning at low fields and a pinning at higher fields.