The different phase transformations occurring in rapidly solidified amorphous ribbons of the Mg83Ni9Y8 (at.%) alloy during heating have been determined. Such transformations involve atomic rearrangement in the amorphous material leading to a first metastable state of extremely fine nanocrystals in the amorphous matrix. Further transformations result in the formation of Mg, Mg2Ni, and an unidentified phase. Heating at temperatures above that of primary crystallization causes additional formation of Mg24Y5 and Ni2Y3 phases. Microhardness measurements were used to evaluate the mechanical properties at room temperature of the structures appearing after each phase transformation. The fine grain size of the crystallized material is the main contribution to harden significantly the totally crystallized alloy. The solid solution of nickel and yttrium in magnesium as well as Mg24Y5 precipitation induce some additional strengthening.