617B is a solid-solution strengthened Ni-based superalloy, a candidate material for 700℃ A-USC power plant. It is always used in solution-anneal at about 700℃. So it is significant to research its microstructure evolution during aging at its service temperature rang form 650℃ to 750℃. In this work, the 617B alloy was exposed prolonged approximately 3000h at 650℃, 700℃, 750℃ respectively. The microstructure evolution had been characterized by optical microscope (OM), field emission scanning electron microscope (FESEM) and energy dispersive spectrometer (EDS). γ" phase and Cr, Mo-rich M23C6 were the major precipitates during aging treatment. γ" phase precipitated at about 650℃,600h and got coarsening with a prolonged time and an increasing temperature. The coarsening of γ" phase met the LSW theory below 700℃ aging but when exposed higher than 750℃, it was priority to coarsen by agglomeration. The carbides along grain boundaries were transformed into continuous network structure by the gradual coarsening of small serration carbides. And carbides within grains increased during aging. When alloy undergone 750℃ long-term aged, there are reactions between carbides, matrix and γ"phase.