Breast cancer that overexpresses human epidermal growth factor receptor 2 (HER2) is called "HER2 positive" breast cancer. The precise diagnosis of HER2 expression in breast cancer increases the chances of patients' survival. Therefore, the development of radionuclide-based diagnostic imaging targeting HER2 will have great potential for translational applications in breast cancer diagnosis and therapeutic evaluation. Aim: This thesis compares the radionuclide-based diagnostic imaging agents (radiotracers) based on positron emission tomography (PET) single photon emission computed tomography (SPECT) targeting HER2 which has promising translational clinical applications. Introduction: It outlines the critical role of HER2 in breast cancer diagnosis and then introduces corresponding imaging agents: molecular imaging probes targeting HER2, radionuclides, chelators, and exceptional diagnostic imaging: functional imaging/molecular imaging (MI). Methods: This dissertation used literature research as the primary method. The information and data were collected, analyzed, evaluated, and summarized. Results: SPECT imaging is more cost-effective than PET imaging; The smaller the size of the molecular probes, the better the imaging contrast; Effective dose of 99mTc is lower than other radionuclides, which have a lower dose burden on patients. [99mTc]-ADAPT6 SPECT imaging has the best imaging properties. Moreover, [99mTc]-(HE)3-G3 SPECT imaging study also completed the Phase I study results and has relatively good imaging properties compared to other agents except [99mTc]-ADAPT6. Conclusion: Among all the radiotracers introduced, [99mTc]-ADAPT6 and [99mTc]-(HE)3-G3 are the most potential agents to enter the next stage of clinical application in the future.