MnBi films were prepared by annealing Bi-Mn composite films. The grain size of the MnBi films was strongly influenced by the temperatureT s of the film substrate during evaporation of Bi and Mn. For 55-nm-thick films, the numberN of MnBi grains per mm2 decreased from 4×106 per mm2 forT s =+90°C to 1 per mm2 forT s =−40°C. The diameters of the largest grains observed were about 5 mm. For interpretation of this result, the formation process of MnBi films was observed magneto-optically and electronmicroscopically. It was found thatN was determined by the structure of Bi in the Bi-Mn composite films and that this strongly depended onT s .