We observed the defect structure in the epilayer grown on an on-axis 4H-SiC substrate using synchrotron x-ray topography. The epilayer contained many 4H-spiral hillocks and 3C-inclusions. The x-ray topographs of the <13> spots in the 3C-structure indicated that the 3C-inclusion comprised {111} twin domains with double-positioning boundaries. Furthermore, stacking faults induced along the <110> direction were observed. The strain fields of the stacking faults strongly affected the structure of the 4H-SiC underlying substrate. In contrast, many 4H-spiral hillocks and basal plane dislocation half-loops were observed in the x-ray topographs of the <0210> in the 4H-structure. The half-loops were developed concentrically around the 3C-inclusion and terminated at the 3C/4H interface. When the concentric half-loops were treated as one group, they had the same Burgers vector, but the Burgers vectors were different for different groups. In the epilayer on the on-axis SiC, the 3C-inclusions, twin domains, stacking faults, spiral hillocks, and half-loops occurred simultaneously.