It was investigated that the microstructure and mechanical property evolution of a supperalloy USC 141 under aging 10000h at 700oC in this paper. The results show that both of the strengths of the alloy tested at room temperature and 700oC were maintained at high level during long-term aging, while the coarsening behavior of carbides along grain boundary, which transformed in a shape of chain generally, leads to decrease plasticity and toughness. During the process of long-term aging，the precipitation of M23C6 showed a trend of slow increasing. Whereas the evolution of M6C type carbide in this alloy consists of three stages. The first stage could be marked as the continuum transformation of M6C to M12C, which was induced by the diffusion of Mo and Ni. At the second stage, the carbide type and the precipitation of carbides changed little. At the third stage, M12C carbides further coarsened afterwards by the diffusion of Mo, and M12C inside grains shows a shape of plate. The microstructure evolution of γ′ precipitates could be divided into three stages as well. The formation and initial grow up fastly of γ′ was resulted from the diffusion of Cr, and the γ′ particles were distributed finely during the first stage. At the second stage, the means size and precipitation of γ′ increase slowly. The last stage was marked by the coarsening behavior of γ′, mainly resulting from the coalescence of adjacent γ′ particles, and finally exhibited higher dispersion level of particle size distribution.