Abstract: By means of a technique of glass encasement on a levitation melting assembly high undercoolings of melt before solidification were obtained for the Nd-Fe-B-Si systems. It is odserved that a transition of growth form from dendritic to cellular occurs as the undercooling is getting higher, and it is a sudden transition, The highest dimensionless undercooling (θ=AT/T_L) obtained for the system excesses 0.3, and X-ray diffraction analysis shows the existence of a metastable phese in the soliditied microstructure in this case.