Spin-polarized density functional theory calculations on bimetallic Co n− 1Mn and Co n− 1V ( n = 2–9) clusters are reported. The clusters exhibit high stability at n = 6. With the exception of CoV, the magnetic moments of Co n , Co n− 1Mn and Co n− 1V increase with size by 3 μ B for n = 2–5 and by 1 μ B afterwards. The magnetic enhancement (reduction) is identified upon Mn (V) substitution, compared to Co n , consistent with recent experimental observation in large-sized clusters. Their different magnetic behavior of Mn and V-substituted clusters results from both differences in magnetic alignment (ferromagnetic in Co n− 1Mn and ferrimagnetic in Co n− 1V) and reduced local moments on Co in Co n− 1V.