Microplastics, or nanoplastics fragmented to sizes in the nanoscale, can easily penetrate living organisms as well as human organs, increasing the risk of toxicity. However, it is challenging to obtain the size of nanoplastics using thermal analysis methods such as pyrolysis gas chromatography/mass spectrometry or thermal desorption-gas chromatography/ mass spectrometry, which are used to analyze nanoplastics. In this study, the coupling effect due to the aggregation of gold nanoparticles (AuNPs) was used to measure the concentration of polystyrene nanoplastics (PSNPs). Experiments were conducted to measure the concentration of PSNPs using an ultraviolet–visible spectrophotometer using the phenomenon that the color of the colloid changes when AuNPs are aggregated. The differences in absorbance before and after aggregation after the addition of NaCl were measured. As a result of the experiment, when 20mM NaCl was added to the solution in which AuNPs and PSNPs were dispersed, the difference in absorbance before and after aggregation and the concentration of PSNPs exhibited high linearity. In addition, 350 and 880 nmsized PSNPs could be distinguished from each other because of their different linearities. The concentration of PSNPs was measured easily and conveniently without requiring a skilled operator, expensive analytical equipment; additionally, the process was not time or labor intensive, and it was shown that particle size can be measured by distinguishing particles of different sizes.