For the past couple of decades, plasmonic nanoparticles such as Au and Ag have employed for wide applications as sensors and probes due to their unique optical properties originated from localized surface plasmon resonance (LSPR). We mainly utilize enhanced scattering light in the visible region. This resonant scattering light is highly sensitive to the surrounding environment; therefore, it can detect minute changes on the surface of nanoparticles. In addition, it provides high optical stability, which gives a great advantage as an imaging probe. Here, we used such enhanced scattering phenomenon of the plasmonic nanoparticles to monitor chemical reactions and biological processes at the nanometer level in real time. In chapter I, background knowledges and recent studies related to LSPR are introduced. In chapter II, the experimental in-situ observations during nanoscale reactions, including (1) Galvanic replacement reaction depending on the temperature and (2) sulfidation by cysteine, using Ag nanocubes as an ideal probe template are demonstrated. Introduction of the flow cell set-up and reaction temperature control allow us to monitor the nanoscale chemical reactions in an environment similar to the actual reaction conditions. In chapter III, we investigated the rotational movement of membrane protein (epidermal growth factor receptor, EGFR) in living cells using a nano-dimer structure assembled with oligonucleotides. This study enables visualization of protein rotation and conformational change, and furthermore provides insights into the activation mechanism.