Using density functional theory, the structures, stabilities and magnetic properties of (FeCr)n (n≤ 6) alloying clusters are systematically investigated. For smaller clusters with n≤3, the results show that the ground-state system possesses collinear antiferromagnetic order. For n≥4 cases, however, the ground-state cluster has noncollinear magnetic order. Therefore, there is a collinear-to-noncollinear magnetic transition at n=4 in (FeCr)n systems. In addition, although the spin-orbit coupling effect of 3d transition metal atom is often weak, the results indicate that the orbital magnetic moments of some certain clusters are significant and important. Finally, the chemical bond of noncollinear magnetic clusters and the physical origin of the magnetic transition are analyzed.