The development of an ultrasensitive nanoprobe is regarded as one of the most promising strategies for the precise diagnosis of tumors. Herein, tanespimycin (17-AAG)-modified polyethylenimine-based gold nanoparticles (AuNPs) co-labeled with a gadolinium (Gd) nanoprobe are constructed for targeted dual-mode computed tomography (CT)/magnetic resonance (MR) imaging. The specific binding of tanespimycin to the N-terminal of heat shock protein 90 (HSP90) endows the modified nanoprobes (NPs) with the ability to up-regulate HSP90 tumor cells and to display excellent X-ray attenuation intensity and T1 MR imaging performance in an orthotopic hepatocellular carcinoma tumor model. Such an ultrasensitive nanoprobe holds enormous promise for highly efficient tumor diagnosis and dual-mode CT/T1 positive MR imaging. This study develops a novel strategy to prepare a multifunctional nanoprobe via polyethylenimine nanotechnology. Using this strategy, various dual-mode or multimode nanoprobes for diagnosing different cancers may be designed.