Amorphous to crystalline transformation of chemically prepared Fe64B36 ultrafine amorphous alloy particles has been investigated by Mossbauer spectroscopy, Brunauer-Emmett-Teller surface area measurements and transmission electron microscopy. Structural relaxation was observed below 350 degrees C, which resulted in narrowing the full width at half maximum for the hyperfine field distribution from 13.0 to 10.6 T, while the average hyperfine field kept unchanged, to be about 20.3 T. Crystallization started on the surface at about 300 degrees C and proceeded into the bulk at about 400 degrees C. Partial crystallization between 400 and 450 degrees C resulted in increasing the average hyperfine field for the remaining Fe-B amorphous matrix to 21.6 T. alpha-Fe and Fe2B were the only iron containing phases related to bulk crystallization, with the latter as a predominant component, accompanied by the segregation of about 19% boron atoms. Above 500 degrees C, sintering of the particles became very remarkable and a solid state reaction between diffusing iron and boron atoms to form Fe2B took place making the spectral area ratio for Fe2B to alpha-Fe components increase accordingly. A locally distorted non-stoichiometric Fe2B quausicrystalline structure for the high boron content sample was proposed.