An investigation on the structure and its decomposition process of an Fe84B16 amorphous alloy by transmission electron microscopy is described. Under an optimum defocus condition, bcc-like microcrystalline domains with a size of about 12 Å were observed by the lattice imaging even in an as-quenched specimen. They were found to grow up to a size about 20 Å on annealing near 300°C. A formation of a halo electron diffraction pattern from the specimens with densely formed microcrystalline domains was explained on the basis of a large lattice distortion of microcrystals. A large amount of inclusion of boron atoms in α-iron precipitates in the early stage of precipitation at 350°C was found by the use of selected area electron diffraction.