Tensile creep tests were conducted at 980°C under a constant stress on a single crystal nickel base superalloy. Some tests were interrupted at different stages during the creep process. The strain-time curves indicated that this alloy exhibited conventional primary, steady-state and tertiary stages at this temperature. The transmission electron microscope (TEM) observations of foils taken from the gauge sections of specimens were made to interpret the microstructural evolution that occurred during the creep process. It was found that the γ′ particles were rafted in the direction perpendicular to the applied stress. The acceleration of the creep rate was related to the change of the dislocation density.