Defects constructing a netlike cell structure in the 3-inch semi-insulating gallium arsenide (SI-GaAs) single crystal were studied by methods of chemical etching, x-ray anomalous transmission topography (XRT) and transmission electron microscope (TEM). The nature and the formation mechanism of these structures were analyzed. It is assumed that the cell structures are made by the clusters of small angle grain boundary caused by the movement and interaction of high density of dislocations. and the cell wall is the typical small angle grain boundary. The phase difference among the small angle grain boundaries increases with the density of dislocations.