<p>To improve the microstructure and physical properties of Fe, Ni, and Co metallic nanoparticles, post-synthesis annealing in three different atmospheres (hydrogen, argon and dry air) was applied. Annealing in hydrogen atmosphere resulted in the formation of pure, metallic nano-sized grains. In the case of the Fe and Co samples, the use of argon and dry air atmospheres during thermal treatment resulted in the formation of oxide phases. For Ni nanoparticles, annealing in argon and dry air did not cause decomposition of the unintended Ni₃B phase which was formed during the synthesis process. Microstructure analysis revealed that all samples, besides the dry air annealed Fe sample, consist of agglomerated nanograins. Annealing of the Fe nanoparticles in dry air resulted in the formation of micrometer-sized structures without clearly defined grain boundaries, making it impossible to compare it to the other Fe samples. All samples, based on recorded temperature dependencies of the magnetization, could be classified as the system of interacting magnetic grains. Magnetization curves measured at room temperature confirm the ferromagnetic behavior of all samples. For the hydrogen-annealed samples the highest saturation magnetization values were registered, and first-order reversal curve diagrams proved the single-domain state of grains in these samples.</p>