Single-domain GdBCO bulk superconductor (20 mm in diameter) has been fabricated by a top-seeding infiltration and growth (TSIG) mathod, it has a new solid phase of [(1-x)(Gd2O3+1.2BaCuO2) + x Ni2O3] (where x =0, 0.02, 0.06, 0.10, 0.14, 0.18, 0.30, 0.50 wt%). Effect of Ni2O3 additions on the growth morphology, microstructure, critical temperature Tc, magnetic levitation force, and trapped flux of single-domain GdBCO bulks have been investigated. Results show that the single-domain GdBCO bulk can be gained when x is in the range of 0-0.50 wt%; and the Gd211 particles are not affected by the Ni2O3 doping in the samples. The Tc of the samples decrease from 92.5 K (x=0 wt%) to 86.5 K (x=0.50 wt%) when x increases from 0 to 0.50 wt%, which is caused by the substitution of Ni3+ for Cu2 +. Both of the levitation force and trapped field of the samples increase first and then decrease with the increase of x; the largest levitation force of 34.2 N is obtained for the samples with x=0.14 wt%, and the largest trapped field of 0.354 T is obtained for the samples with x=0.10 wt%. The change of the levitation force and trapped field of the samples is closely related to the doping content x. As is known, the doping of Ni2O3 can result in substitution of Ni3+ for Cu2+ at its site in GdBCO crystals, which can reduce the critical temperature Tc of the samples; although Tc and the physical properties of the samples is reduced with the increase in the doping amount of Ni2O3, but at the same time, the substitutions of Ni3 + for Cu2 + in GdBCO crystals can produce local lattice distortions, which can act as magnetic flux pinning centers to improve the properties of the samples. The highest Tc is obtained in the samples without any Ni2O3 additions (x=0), but the magnetic flux pinning force of the samples is weak, so both of the levitation force and trapped field of the samples are relatively lower. When the doping content x ≤ 0.14 wt%, although the Tc is reduced slightly, it still has a value higher than 90 K; and the magnetic flux pinning force in the samples, due to the substitutions of Ni3+ for Cu2 +, would increase with the increase of doping content x, and result in an enhancement of levitation force and trapped field. When the doping content x is greater than 0.14 wt%, the magnetic flux pinning force of the samples is still increasing with the increase of x, but the Tc of the sample is significantly reduced and even less than 90 K, and finally result in an decrease of levitation force and trapped field. Only when the doping amount of Ni2O3 is appropriate, both of Tc and magnetic flux pinning force are of a relative optimal value, and lead to an enhancement of levitation force and trapped field.