Abstract: Interfacial dislocations found in single crystal superalloys after long term thermal aging have an important effect on mechanical properties. Long term thermal aging tests for DD5 single crystal superalloy were carded out at 1,100 degrees C for 20, 100, 200, 500 and 1000 h, and then cooled by air. The effect of long term thermal aging on the dislocation networks at the gamma/gamma' interfaces was investigated by FE-SEM. Results showed that during the long term thermal aging at 1,100 degrees C, misfit dislocations formed firstly and then reorientation in the (001) interfacial planes occurred. Different types of square or rectangular dislocation network form by dislocation reaction. Square dislocation networks consisting of four groups of dislocations can transform into octagonal dislocation networks, and then form another square dislocation network by dislocation reaction. Rectangular dislocation networks can also transform into hexagonal dislocation networks. The interfacial dislocation networks promote the gamma' phase rafting process. The dislocation networks spacings become smaller and smaller, leading to the effective lattice misfit increasing from -0.10% to -0.32%.