A model of 5.3° low angle grain boundaries in steel is set up by using elastict heory of dislocations.The first-principle calculations of energies and the elect ronic structures of impurities for a 5.3° low angle grain boundary (GB) in stee l have revealed important features of the impurity effect.In Fe,impurities such as S,P weaken the intergranular cohesion,resulting in ‘loosening’ of the GB.Th e presence of B,C,N on the contrary enhances the GB.Boron plays a dual role in s teel,not only does it presence at GBs enhance the intergranular cohesion,but als o accomplishes ‘site competition cleaning’ by displacing the other impurity at oms off the GB.