The isovector giant dipole resonance (GDR) of an &agr;‐cluster in6Li and7Li, and their analogs in6,7He and6,7Be were searched for by using the6,7Li(p,p′),6,7Li(3He,t) and6,7Li(7Li,7Be) reactions at the incident energies of 300, 450 and 455 MeV, respectively. New resonances were found for the first time atEx=27.0±1.5 MeV in6Li and atEx=29.0±1.5 MeV in7Li, both with widths (FWHM) of 12±2 MeV. In6He,6Be,7He and7Be resonances with widths of 12±2 MeV were observed via the charge exchange reactions atEx=24.0±1.5, 23.5±1.5, 18.0±1.5, and 28.0±1.5 MeV, respectively. All of these resonances were assigned to be dipole resonances based on the measured angular distributions of differential cross sections. The resonance shapes were reproduced well with the resonance shape of the GDR of4He reported in the (&ggr;,n) reaction. The averaged ratios of the cross sections for these resonances in A=6 to those in A=7 for each isotope were estimated to be nealy unity, 1.2±0.3. The averaged value of the respective cross section ratios of these resonances to the GDR in their respective target nuclei was 0.44±0.08, which was consistent with the ratios of the cross sections of the GDR in4He to those of the GDR in6,7Li reported in the photo‐nuclear reactions. The excitation energies of these dipole resonances in A=6 and 7 nuclei relative to the separation energies of an &agr;‐particle in6Li and7Li, respectively, agreed well with the excitation energy of the GDR in4He (≈26 MeV). We conclude that the resonances observed in6,7Li are the GDR of the &agr;‐cluster and the resonances in6,7He and6,7Be are their analogs. © 2004 American Institute of Physics