The work shows how the concentration of Ni nanoparticles with mean sizes of 6 ± 2.3, 58 ± 24, and 171 ± 65 nm influences the germination of wheat seedlings (Triticum aestivum L.). At a concentration of 3–30 mg/L, seedling root development weakly depends on the presence of particles in the germination medium, but the growth of shoots is stimulated (shoot length increases by 18–31%). With an increase in the concentration from 30 to 300 mg/L, the addition of larger particles does not inhibit seedling germination, while in the presence of 6-nm nanoparticles the length of roots and shoots decreases by 23 and 61%, respectively, compared to the control. It is also shown that the biological properties in the germination medium are influenced not by aggregation, but by the initial particle size.