Abstract: The microstructure evolution of single crystal tungsten working at high temperature for 1 000 h was investigated using optical microscopy method in combination with electron backscatter diffraction (EBSD) technique in this study. The experimental results show that different crystal planes correspond with different specific etching morphology. The etching morphology of 110 crystal plane consists of parallel stepped structure and the etching morphology of 112 crystal plane consists of stacked cubic structure. The etching morphology of single crystal tungsten along the axial direction after serving is not uniform. Compared with the center zones, the etching morphology at the end zones of the emitter has remained relatively intact. In addition, small grains were observed along the axial direction and the average grain size of them was about 200 μm. Some subgrains whose edges were not clear were also observed. The grain size of the small grains is larger than that of the subgrains. Surface indices of all small grains vary from each other and show no apparent preferred orientation. However, all the misorientation angles between the small grains and matrix were analyzed and the results show that they are all smaller than 30°.