The effect of addition of Mg on the creep behaviour of Ni-base wrought superalloy has been investigated. It was found that the trace Mg added may cause the alteration of continueous lath carbide into discontinueous massive one at the grain boundaries. It may also promote the spheroidization of the grain boundary carbide, decrease the diffusion coefficient of grain boundary and increase the activation energy for creep. The grain boundary energy and the boundary energy between grain boundary carbide and matrix are decreased and bonding strength of interface increased. Thus, the creep property and the creep-rupture life as well as the stress exponent of creep of the superalloy may be significantly improved by the impediment of the boundary sliding and the prevention of the nucleation and propagation of cracks.