The defects in the transition from square to square grid states are studied in a dielectric barrier discharge. It is found that there exist two kinds of defects in this transition, which are angle defect and dislocation defect. In order to investigate the formation mechanism of the defects, the light emissions from different filaments are measured, which show that the light intensity of filament in the middle of the octagonal cell is larger than that in the side. By introducing the quasi-potential field, the interactions between filaments as well as their influences on the transition between different structures are studied. From above analyses, the defects that could emerge in the transition are suggested, which are in good agreement with that observed in experiments.