The origin and the formation mechanism of a surface morphological defect in 4H-SiC epilayers are reported. The defect appears on the surface of an epilayer as an inclined line-like feature at an angle of +/- 80 degrees to the step-flow direction [ 11 2 over bar 0 ] . The defect is confirmed to originate from a threading screw dislocation intersecting the surface and its orientation is controlled by the sign of the Burgers vector of the dislocation. The defect forms through the interaction of local spiral growth associated with threading screw dislocations and step-flow growth related to the substrate offcut. The defect mainly appears in the epilayers grown through chloride-based chemistry, where in situ surface preparation of the substrate is performed in H-2 + HCl at a relatively high temperature.