Fe100 - xNix alloys of ultrafine particle with the average grain size of about 10 nm were synthesized by mechanically alloying process. The samples were investigated by X-ray diffraction and measurements of the saturation magnetization and coercivity force. Both b. c.c and f. c. c phase exist within a wide range for Fe100-x Nix, while x≤45. The effective magnetic anisotropy Ke was measured by applying the law of approach to saturation. The value of Ke decreases with an increase of Ni content. It is noticed that the strain anisotropy makes a large contribution to the magnetic anisotropy. The estimation of grain size leads to the determination of the single domain critical size and domain wall energy. The exchange stiffness and exchange integral deduced from the relationship between the effective magnetic anisotropy and domain wall energy are in agreement with that calculated by other methods.