 The aim of this study was to examine the effects of nickel oxide nanoparticles on the activity and thermal stability of lysozyme obtained from Rutilus frisii kutum. The synthesis of the nanoparticles was performed by the chemical co-precipitation method. To evaluate the synthesis process, X-ray diffraction, scanning electron microscope, and Fourier transform infrared spectroscopy were evaluated. The size of the nanoparticles was in the range of 20–30 nm with a spherical morphology and gray–black color. To determine thermal stability, the fraction unfolded curve and ΔG° as a function of lysozyme temperature in the absence and presence of the nanoparticles were obtained. With respect to Tm at ΔG° = 0, we obtained Tm of lysozyme from 314 K in the absence of the nanoparticles to 332 K in the presence of 0.0001 g/cm3 nickel oxide nanoparticles. Therefore, thermal stability of lysozyme was increased with the rise of nanoparticles’ concentrations. The reversible unfolding as a result of heating lysozyme was observed at Tm = 318 K. Kinetic parameters, Km and Vmax, of the enzyme were also determined from 0.007 g/cm3 and 172 units/min in the absence of the nanoparticles to 0.005 g/cm3 and 104.166 units/min in the presence of 0.0001 g/cm3 nickel oxide nanoparticles, respectively. The emission intensity of lysozyme decreased in the presence of the nanoparticles, while its absorption intensity increased without any shift at 340 and 280 nm, respectively. Consequently, the nickel oxide nanoparticles can interact with lysozyme that results in decreasing its enzymatic activity due to changes in its active site.