Effects of solid solution treatment on microstructure and mechanical property of AM60B alloy with rare earth additions were studied. The experimental results indicated that the ultimate tensile strength and yield strength of alloys were obviously improved by addition of RE no more than 1.6%, while their elongation were not decreased much, microstructure of AM60B+xRE alloys consisted of following phases: delta (Mg) solid solution, rod-like Al11RE3 phase, particle-like Al10Ce2Mn7 phase, and net-like and/or insular Mg17Al12 phase. After solid-solution treated(T4) for 20hr, 35hr, and 50hr at 410degreesC respectively, the Mg17Al12 phase almost dissolved in Mg matrix, but the rare earth compounds Al11RE3 and Al10Ce2Mn7 phases didn't dissolve and their morphology was slightly changed. When the content of RE reached 1.6%, the alloys' UTS reached peak.