Superconducting bulks of MgB2 with addition of Sb2O3 and Sb with different stoichiometric compositions ((MgB2) + (Sb2O3) (x) , x = 0.0025, 0.005, 0.015, and (MgB2) + (Sb)(y), y = 0.01) were obtained by the Spark Plasma Sintering (SPS) technique. All added samples have high density, above 95% and critical temperature, T (c), of 38.1-38.6 K. This result and XRD data suggest that Sb does not enter the lattice of MgB2. Impurity phases are Mg3Sb2, MgO, and MgB4. The optimum addition is Sb2O3 for x = 0.005. This sample shows the critical current density, J (c)(5 K, 0 T) = 4 x 10(5) A/cm(2) and J (c)(5 K, 7 T) = 6 x 10(2) A/cm(2), while the irreversibility field, H (irr) (5 K, 100 A/cm(2)) = 8.23 T. Indicated values of J (c) and H (irr) are higher than for the pristine sample. The mechanism of J (c) and H (irr) increase in the Sb2O3 added samples is complex and composed of opposite effects most probably involving morphology elements, the presence of nano metric MgB4 and the indirect influence of oxygen or oxygen and Sb. Crystallite size of MgB2 is decreasing when Sb-based additions are introduced and the effect is stronger for the Sb-metal addition. The sample with Sb-metal addition does not improve J (c) and H (irr) when compared with pristine sample.