In this paper, the stress-rupture tests of a low Re-containing single crystal alloy IC21 before and after thermal exposure at 1100 °C for various periods of time were conducted under the test condition of 1100 °C/137 MPa, and the microstructure of the tested specimens was characterized by SEM and TEM. The experimental results showed that the stress rupture life of this alloy was over 150 h after the standard heat treatment of 1320 °C, 10 h/AC+870, 32 h/AC, however the stress rupture life decreased with the increase of exposure time due to the microstructure degradation. The TEM analysis revealed that the interface mismatch dislocation networks were well established. It was observed that these mismatch networks could form at 1100 °C even after thermal exposure for 1 h without the external stress, which is quite different from that in the traditional single crystal superalloys.