Experiments were conducted to evaluate the grain refinement and thermal stability of ultra-fine grained Al-4Mg alloy introduced by equal-channel angular pressing (ECAP) at 473 K. The results show that the intensities of X-ray (111/222) and (200/400) peaks for the alloy processed by ECAP decrease significantly and the peak widths of half height become broadening compared with the corresponding value in the annealed alloy. The microstructure of 2 passes ECAPed alloy consists of both elongated and equiaxed subgrains. The residual strain in the alloy increases with increasing passes numbers, that appears as increasing dislocation density and lattice constant of matrix. An equiaxed ultra-fine grained structure of ～0.2μm is obtained in the present alloy after 8 passes. The ultra-fine grains are stable below 523 K, because the alloy retains extremely fine grain size of ～1μm after static annealing at 523 K for1 h.