Tailoring the structure of nanocrystalline microstructures is an important step toward controlled design of novel nanostructured materials and devices. We demonstrate how the nanocrystalline microstructure of Co-rich ribbons can be tuned by annealing under magnetic field. The intensity of the field allows controlling different degrees of order at annealing temperatures corresponding to the first stages of the nanocrystallization process. The energy barrier for nucleation is directionally affected by the applied field. The influence of grains assembling on exchange coupling between grains has been analyzed by means of magnetic domains observation and magnetic characterization by means of a hysteresis loop.