Defect formation during the ion implantation/annealing process in 4H-SiC epilayers is investigated by synchrotron reflection X-ray topography. The 4H-SiC epilayers are subjected to an activation annealing process after Aluminum ions being implanted in the epilayers. The formation modes of extended defects induced by the implantation/annealing process are classified into the migration of preexisting dislocations and the generation of new dislocations/stacking faults. The migration of preexisting basal plane dislocations (BPDs) takes place corresponding to the ion implantation interface or the epilayer/substrate interface. The generation of new dislocations/stacking faults is confirmed as the formation of Shockley faults near the surface of the epilayer and BPD half-loops in the epilayer.