We have successfully demonstrated a novel sensing technique for monitoring the variation of solution concentrations and measuring the effective dielectric constant in a medium by means of an ultra-small and label-free nanosensor, the mechanism of which is based on the localized surface plasmon resonance (LSPR) of gold nanorods. The nanorods are fabricated in a narrow size distribution, which is characterized by transmission electron microscopy and optical absorption spectroscopy. In addition, we employ a simple analytical calculation to examine the LSPR band of the absorption spectrum, which provides excellent consistency with aspect ratio. The plasmonic sensing is performed by detecting the diffusion process and saturation concentration of hexadecyltrimethylammonium bromide in water, and tracing the effective dielectric constants of the medium simultaneously. This promising sensing and analytical technique can be easily used for investigating the nano-scale variations of mixing or reaction process in a micro/nanofluidic channel or the biological interaction in the cytoplasm of the cell.