Tensile properties at room temperature of a new casting Ni-base superalloy during aging at 800 ℃ for 0-1000 h were investigated.During aging,granular M23C6 carbides presented at grain boundaries and kept growing from dispersed particles to continuous networks.The γ'phase significantly coarsened,with the morphology of some γ'phase changed from spherical to rounded cubic shape after 1000 h.Three deformation mechanisms in relation to the γ'diameter (dγ') were identified: (i) weakly coupled dislocations (WCD) connected by anti-phase boundary (APB) traveled in pair across the γ/γ'structure when dγ'was small in the under-aged alloys;(ii) strongly coupled dislocations (SCD) with reduced spacing compared to (i) sheared γ'phase when dγ'increased in the over-aged alloys;(iii) dislocations occasionally by-passed γ'phase when dγ'was larger than 97 nm after aging for more than 300 h.The alloy obtained the peak strength when 20 h-aged with dγ'=44 nm which was in the transition between (i) and (ii).The aging-induced variation in yield strength was correlated to the coarsening of γ'phase using a theoretical model of precipitation strengthening in terms of the formation of APB.The calculated results suggested that the γ'phase with a volume fraction of 23％ contributed more than 61％ of the peak-aged yield strength.Observation after fracture revealed that the alloys usually fractured at grain boundaries.High stress concentration around carbides resulted in cracks by carbides self-cracking and the initiation of cavities.The undesirable agglomeration of M23C6 at grain boundaries was harmful to the properties of the overaged alloys.