At intermediate and high temperatures (700–1000°C) primary creep of nickel base superalloy single crystals appears as an incubation period (part I) with a very low creep rate ( ge < 10 −5/ h). Following a transition period where ge increases rapidly, secondary creep sets in with a constant creep rate of the order ge ∼- 10 −4/ h. Secondary creep is analized in terms of effective stress σ e = σ a − σ i ( σ a , T) and effective creep parameters Q e and n e are estimated Experimental determinations of the apparent creep parameters yield n a = 11 and Q a ∼- 135 kcal/mole in this stage. For creep rates of the order of 10 −4/h, internal stress measurements, as estimated from stress dip tests, point to values of σ i in the vicinity of 0.85−0.95 σ a . When performed under various temperature and applied stress conditions, they lead to the determination of σ isu∗ = ( ∂σ i ∂T) σ a equal to −0.5 MPa/°C and of σ' i = ( ∂σ i ∂σ a ) T equal to 0.2. An effective activation energy of about 60 kcal/mole and an effective stress exponent of the order of 1 are derived from these results. Direct observation of dislocation and precipitate structures in relation with effective creep parameters during secondary creep suggests a high temperature creep model for nickel base superalloys in the range of temperature and applied stress where shearing of the γ' phase does not control the straining process. During secondary creep, strain is mainly the result of climb of dense edge dislocation networks in the γ-γ' interfaces: the dislocation density (− 10 9/cm 2) is kept constant by the glide of screw dislocations between γ' precipitates. Effective creep parameters are typical of a diffusion controlled process: edge dislocations are climbing in the γ-γ' interfaces. This mechanism is responsible for the mass transport leading to the oriented coarsening of the hardening phase taking place during high temperature deformation and also for the typically unstable plastic flow usually observed in this type of high performance material.