The preferred site of Mg segregation atΣ = 11(113) grain boundaries in Al and the effects of Mg segregation on grain boundary cohesion havebeen investigated through first-principle pseudopotential total energy calculations. Theresults show that the Mg atom prefers to occupy the ‘looser’ site at the grain boundary ofAl but not the ‘tighter’ site. Furthermore, on basis of the thermodynamic theory of Riceand Wang, we have studied the effect of Mg segregation on the grain boundary cohesion ofAl. Our total energy calculations show that Mg behaves as an embrittler. Additionally, thecohesive energies for a pure Al boundary and an Al boundary with Mg segregation havebeen calculated in this paper. The calculated results indicate that the cohesion of an Algrain boundary with Mg segregation is weaker than that of a pure Al grain boundary. Thetotal charge density of the gain boundary also demonstrates that the Mg atom formsweaker metallic bonds with neighbouring Al atoms in the grain boundary region.