The composition dependence of magnetic properties in ultrafine (Fe1−xNix)76Si8B16amorphous particles has been investigated. Samples were prepared by sputter deposition onto sputter‐etched polyimide substrates. The particle diameter was held constant at about 800 A˚, and theL/Dratio was approximately 4. The room‐temperature saturation magnetization (Ms) decreases with increasing Ni content and extrapolates to zero at aboutx=0.9. All the samples showed positive anisotropy (Ku). This anisotropy varied linearly withM2s, suggesting that its origin is largely the shape anisotropy of the particles. Furthermore, the measured perpendicular coercive field varied linearly withKu/Ms, as predicted by a simple Stoner–Wohlfarth single domain model. However, the absolute value ofHcand its angular dependence cannot be explained by the coherent rotation model.