In this study, three kinds of YBCO samples which are named Y1, Y2 and Y3 were fabricated by a melt-powder-melt-growth (MPMG) method. The Y1 sample was placed into a platinum (Pt) crucible without Y2O3, the Y2 sample was located on a Al2O3 crucible with a freely poured Y2O3 powder and the Y3 sample was located on a Al2O3 crucible with a 1-mm-thick buffer layer of Y2O3. YBCO samples were investigated by magnetoresistivity (-T) measurements in dc magnetic fields (parallel to the c-axis) up to 5 T. The effect of the Y2O3 layer on the activation energy and irreversible flux of the samples was studied. The activation energies (U) were determined using the Arrhenius activation energy law from -T. The power law relationship for U with H- was investigated. was calculated to find out which defects were dominant in the samples. Irreversibility fields (H-irr) and upper critical fields (H-c2) were obtained using 10 and 90% criteria of the normal-state resistivity value from -T curves. Irreversibility lines (ILs) were estimated from the equation H-irr approximate to (1 - T-irr(H)/T-irr(0))(n). The fitting results to giant flux creep and vortex glass models were discussed.