The influence on the crystallization and mechanical properties of partial replacement of cerium-rich mischmetal (CeMM) by yttrium in amorphous alloys, whose compositions were close to Mg80Ni10(Y + CeMM)10 (at.%) stoichiometry, has been evaluated. Yttrium addition induces substantial microstructural changes during the crystallization from the amorphous state. The microstructure consisted of a matrix embedding some intermetallic phases, which depend on yttrium and CeMM additions. The matrix is a long period ordered structure for the alloy containing exclusively yttrium whereas the matrix was the intermetallic Mg17RE2 and/or Mg12RE phase for the CeMM-containing alloys. Yttrium improves the mechanical strength of the crystallised alloys once the long period ordered structure is developed.