Nanocrystalline and amorphous Mg-Ni-Cu-Mn alloys with nominal compositions of Mg-5 wt.% Ni-5 wt.% Cu-x wt.% Mn (x = 0, 1, 3) were synthesized by the melt spinning technique. The structure of the alloys was characterized by scanning electron microscopy and X-ray diffraction analysis. The phase transition during the solidification process was studied by differential scanning calorimetry under an Argon atmosphere. The wheel speed, which is closely associated with the cooling rate, was varied in the production of the melt-spun ribbons. The resultant melt-spun ribbons had various microstructures ranging from dendrite to equiaxed polygonal grains, and finally to an amorphous structure, depending on the wheel speed. The wheel side of the ribbons exhibited a finer microstructure than those on the free or air sides exposed to the atmosphere. Experimental results showed that the mechanical properties of the Mg-Ni-Cu-Mn alloys were enhanced, which can be attributed to significant changes in the microstructure. It was found that the microhardness values of the melt-spun ribbons were strongly dependent on the solidification rate and Mn content. The highest microhardness values were observed on the wheel side of the ribbons. (C) 2013 Elsevier B.V. All rights reserved.