Magnetic-field-induced domain reversal mechanisms of antiferromagnetic (AF) NiO thin films are investigated by spin Hall magnetoresistance (SMR) measurements. The field strength dependence of SMR amplitudes is measured in epitaxial and polycrystalline NiO films. From the field-dependent behavior of SMR amplitude, two distinct domain reversal mechanisms are found for those NiO films. In the epitaxial films, the conventional monodomain formation against the destressing field due to the magneto-elastic coupling is observed. On the other hand, the thermally assisted domain reversal is dominant in the polycrystalline films. Based on our thermal assisted model, the effective values of domain pinning potential and the number of spins contributing to domain reversal in polycrystalline films are determined. These values are quite important to design AF spin memory devices. This study contributes to building a method to determine the key parameters in AF spintronics with polycrystalline thin films, which are free from the lattice mismatching problem.