Although many breast and lung cancers overexpress human epidermal growth factor receptor-2 (HER-2), no methods currently exist for effective and early detection of HER-2-positive cancers. To address this issue, we designed and synthesized dendrimer-based novel nano-imaging agents that contain gold nanoparticles (AuNPs) and gadolinium (Gd), conjugated with the humanized anti-HER-2 antibody (Herceptin). Generation 5 (G5) polyamidoamine (PAMAM) dendrimers were selected as the backbone for the nano-imaging agents due to their unique size, high ratio of surface functional groups and bio-functionality. We modified G5 PAMAM dendrimer surface with PEG and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelators to encapsulate AuNPs and complex Gd. These dendrimer entrapped AuNPs were further conjugated with Herceptin through copper-catalyzed azide- alkyne click reaction to construct the nano-imaging agent Au-G5-Gd-Herceptin. The targeted nano-imaging agent bound selectively to HER-2 overexpressing cell lines, with subsequent internalization into the cells. More importantly, non-targeted nano-imaging agent neither bound nor internalized into cells overexpressing HER-2. These results suggest that our approach could provide a platform to develop nano-diagnostic agents or nano-therapeutic agents for early detection and treatment of HER-2-positive cancers.