The expansion of double-Shockley stacking faults (DSFs) in 4H-SiC was investigated by a photoluminescence (PL) imaging technique. To observe DSFs nondestructively, heavily-nitrogen-doped epilayers were prepared to be used as specimens and the PL technique was applied for the observation. The size and shape of the DSFs in the epilayer were clearly distinguished in the two dimensional PL images, although it is normally difficult to observe luminescence from DSFs in substrates grown by the sublimation method. The nucleation and expansion of the DSFs were tracked in the course of successive high-temperature annealing. Dislocation velocities of Shockley partials along the edge of the DSFs were evaluated from the PL observations. Activation energy for the dislocation glide was estimated from the temperature dependence of the velocities. The nucleation sites of the DSFs are also discussed. •DSFs in heavily-nitrogen-doped epilayer were clearly observed in PL images.•Behavior of the DSF during high-temperature annealing was tracked.•Expansion velocity increased as temperature was elevated.•Activation energy for the dislocation glide was obtained.•Mechanical stress and mismatch stress seemed to trigger DSF nucleation.