[en] Comparative analysis between Zr-rich Zr₅₀Cu₄₅Al₅ and Cu-rich Cu₅₀Zr₄₅Al₅ metallic glasses (MGs) is extensively performed to locate the key structural motifs accounting for their difference of glass forming ability. Here we adopt ab initio molecular dynamics simulations to investigate the local atomic structures of Zr₅₀Cu₄₅Al₅ and Cu₅₀Zr₄₅Al₅ MGs. A high content of icosahedral-related (full and distorted) orders was found in both samples, while in the Zr-rich MG full icosahedrons $⟨<!--\; ???\; -->0,0,12,0⟩<!--\; ???\; -->$ is dominant, and in the Cu-rich one the distorted icosahedral orders, especially $⟨<!--\; ???\; -->0,2,8,2⟩<!--\; ???\; -->$ and $⟨<!--\; ???\; -->0,2,8,1⟩<!--\; ???\; -->$, are prominent. And the $⟨<!--\; ???\; -->0,2,8,2⟩<!--\; ???\; -->$ polyhedra in Cu₅₀Zr₄₅Al₅ MG mainly originate from Al-centered clusters, while the $⟨<!--\; ???\; -->0,0,12,0⟩<!--\; ???\; -->$ in Zr₅₀Cu₄₅Al₅ derives from both Cu-centered clusters and Al-centered clusters. These difference may be ascribed to the atomic size difference and chemical property between Cu and Zr atoms. The relatively large size of Zr and large negative heat of mixing between Zr and Al atoms, enhancing the packing density and stability of metallic glass system, may be responsible for the higher glass forming ability of Zr₅₀Cu₄₅Al₅. (paper)