The specimens of single-crystal aluminum were irradiated with high current pulsed electron beam (HCPEB). The vacancy cluster defect microstructure has been investigated by using transmission electron microscopy (TEM). The results show that three types of vacancy clusters including dislocation loop, void and even stacking fault tetrahedron (SFT), which are not normally formed in metals with high SFT formation energy, can be formed in single-crystal aluminum specimens irradiated with HCPEB. The nucleation process of three types of vacancy clusters do not appear at the same time. There is a close relationship between the three types of vacancy clusters. Based on the experimental results, a possible mechanism of SFT formation and evolution was presented.