Rapidly solidified amorphous Mg-23.5Ni (wt.%) ribbons were crystallized at 300 and 400 degrees C for 90 min. After annealing at 300 degrees C the microstructure was heterogeneous, consisting of rounded eutectic-lamellar domains, which contained magnesium grains smaller than 500 nm. In the case of ribbons annealed at 400 degrees C the microstructure, however, was homogenous, and composed of well-formed magnesium grains and Mg2Ni particles. At room temperature both crystallized materials were brittle due to the high volume fraction of Mg2Ni particles, but they exhibited some ductility with increasing test temperature. Above 200 degrees C, the microstructure of the ribbons annealed at 300 degrees C was characterised by the formation of particle free zones during the tensile test. This structure was not observed in the material annealed at 400 degrees C. Deformation behaviour and changes in the microstructure during plastic flow of both crystallized materials were explained according to grain boundary sliding mechanisms.