Silicon carbide (SiC) substrates with chemical vapor deposition (CVD) grown epilayers have been characterized by synchrotron white beam X-ray topography (SWBXT). Large numbers of circular basal plane dislocation loops (BPDs) were observed in the substrate which were anchored by threading screw dislocations (SDs). Threading edge dislocations (TEDs) are not observed to play an important role in the multiplication of BPDs. A SD-assisted “conservative climb” model is proposed to explain the multiplication of BPDs during growth and/or post-growth processes. BPDs are shown to multiply on adjacent parallel basal planes via single SD-assisted as well as opposite sign SD-pair-assisted “conservative climb”.