A new nickel-base superalloy Ni-25Cr-20Co-0.5Mo-2Nb-1.7Ti-0.8Al has just developed by Special Metals Corporation in the application to ultra-supercritical boilers with steam temperatures up to 700°C. The structure stability of the alloy was studied in detail in combination of SEM, TEM, XRD and micro-chemical phase analyses. Experimental results show that the most important structure instability is as follows: 1) g¢ coarsening; 2) g¢ to h transformation; 3) G phase formation. The phase computation by means of Thermo-Calc has been adopted in chemical composition modification for structure stability improvement. Two suggested new modified alloys in adjustment of the Al and Ti contents and in control of Si level were designed and melted for experimental study. These 2 modified new alloys exhibit more stable structure at 760oC long time aging. A bright perspective has been shown for the development of this new alloy in the near future.