In this report, spectrophotometric, kinetic, and structural analysis has been carried out for the formation of cobalt nanoparticles (CoNPs) using 4-aminophenol as reducing agent. The localized surface plasmon resonance band (SPR) of cobalt nanoparticles in the UV-vis spectrum is used to determine the rate of formation of cobalt nanoparticles and assess the beginning of the oxidation process. The effect of cobalt nitrate, 4-aminophenol and TTAB concentration was investigated on the growth rate of cobalt nanoparticles. CoNPs were characterized by means of UV-vis, field emission scanning electron microscope (FESEM), and energy dispersive spectroscopy (EDS). The data obtained in this work provide valuable information on the rate of reactions at the nanoscale.