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This work is a continuation of the characterization study of nickel oxide (NiO) nanoparticles (NPs) doped with transition metals (TM) like Mn, Fe and Zn, synthesized by the co-precipitation method. In this part, the effect of TM doping on the physical and mechanical properties of the NiO NPs is examined using a digital Vickers microhardness tester. The microhardness measurement results enable us to evaluate the important mechanical parameters including Vickers microhardness (HV), Young's modulus (E), yield strength (Y) and fracture toughness (KIC), responsible for practical industrial applications. It was found that all the properties given above strongly depend on the type of TM doping element. Specifically, Vickers microhardness Hv values of the samples studied in this work were found to decrease by doping with Mn, Fe, and Zn. Moreover, the normal Indentation Size Effect (ISE) behavior was observed. The experimental results were analyzed according to Meyer's Law, Proportional Sample Resistance (PSR), Elastic/Plastic Deformation (EPD) models, and Hays–Kendall (HK) approach. As a result, the Hays–Kendall (HK) approach is found to be the best model for the load-independent microhardness determination of the studied samples.