Effect of size and volume fraction of γ′ particles and the coherency strain due to the γ-γ′ mismatch on tensile deformation characteristics was investigated with Udimet 520 and Ni-6.38%Al alloy and the underlying mechanisms were discussed. The observed transition in the mode of dislocation-γ′ interaction from shearing to by-passing with proceeding of the age-hardening was found to correlate closely with variations of the proof stress, strain-hardening coefficient and internal friction. A quantitative analysis of the age-hardening of Udimet 520 on the basis of a theoretical model of shearing of γ′ particles by paired dislocations revealed that some modification of the existing theories is necessary.