The effects of long-term aging （800-900 ℃, （1-10）×10^3 h） on the γ＇ coarsening characteristics and the mechanical properties of a corrosion resistant Ni base superalloy K44 have been investigated. SEM observation reveals that the coarsening kinetics of γ＇ follows LSW theory at different temperatures and the activation energy for γ＇ growth is 255 kJ/mol. Morphological instability occured in relatively small γ＇ precipitates during aging process. At room temperature and 900 ℃, the yield strength decreases with increasing aging temperature and aging time, while a fluctuation of elongation was observed. The dependence of the yield stress on particle size is explained by Labusch-Schwarz hardening theory rather than classical strengthening theory on coherency and mismatch strengthening. Shearing of strong-coupled dislocations is dominant strengthening mechanism.