We report the results of the local structural evaluation and mechanism of QC formation in the Zr 70 Pd 30 and Zr 80 Pt 20 glassy alloys. Voronoi analysis indicates the difference of local environment between two alloys. The perfect icosahedron frequently exists around Zr atom and major polyhedra have prism-like structure around Pd in Zr 70 Pd 30 . In contrast, icosahedral-like distorted polyhedra formation is favorable around Pt as well as Zr in Zr 80 Pt 20 . It is therefore, concluded that the quasicrystallization originates from the medium-range order based on the Zr-centered perfect icosahedron and the Pd-centered prism-like ones remain during the QC phase formation in Zr 70 Pd 30 . Icosahedral-like local structure around Zr and Pt might contribute together to the nucleation of QC phase in Zr 80 Pt 20 . This feature with a different mechanism of QC formation in the two alloys may correlate to the difference of solute concentration and the structure of stable crystalline phase after the decomposition of QC phase.