The composition and size dependence of the magnetic properties of crystalline Fe‐Co and Niultrafine particles has been investigated. Samples were prepared by sputter deposition onto sputter‐etched polyimide substrates. Little difference between the magnetic properties of crystalline and amorphous particles was found in Fe‐rich alloys, but we found a greater influence of magnetocrystalline anisotropy in Co‐rich alloys. When the particles are larger than 2500 Å, the samples have negative anisotropyK u (easy direction parallel to the substrate plane). As the particle diameter decreases, K u becomes positive, and the coercive fieldH c increases from a few hundred oersteds to a maximum of more than 1000 Oe. Below a particle diameter of about 1000 Å, M s , K u , and H c all drop rapidly toward zero, presumably due to the appearance of superparamagnetism.Annealing above about 200 °C leads to significant increases in H c , K u , and M s , and also to significant changes in the line shapes of the x‐ray diffraction patterns, as a result of stress relaxation. A low‐temperature diffusion process leading to changes in the morphology of the subparticles may also be a cause of the increment of magnetic properties on annealing.