This study investigates the determination of gold nanoparticle sizes using UV-Vis spectroscopy, a powerful technique for analyzing the optical properties of nanomaterial. Gold nanoparticles exhibit unique optical characteristics due to their localized surface Plasmon resonance (LSPR), which is sensitive to particle size, shape and surrounding environment. In this work, gold nanoparticles were synthesized through chemical methods and characterized their size by analyzing the absorbance spectra obtained from UV-Vis measurements. The experimental absorbance data were compared with theoretical predictions based on Mie theory. The review demonstrate a strong correlation between the experimental and theoretical absorbance values, particularly at the characteristic peak around 500 nm, confirming the effectiveness of UV-Vis spectroscopy in nanoparticle size determination. This work contributes to the understanding of gold nanoparticles' optical properties and their potential applications in various fields, including biomedical imaging, drug delivery and sensing technologies. The amount of citrate is increased the absorption spectra shows better agreement with Mie Model Simulations. The size of nanoparticle increased the wave length also increased which was directly proportional to each other but the absorbance is decreased