The influence of pH values on the gold nanoparticles' growth induced by UV irradiation is investigated. Citric acid is found to act as a cappingligand for stabilizing the metal nanoparticles as well as a photoreduction agent for the metal ions. The photoreduction of Au (Ⅲ) ions and thegrowth processes of nanoparticles at different pH values are characterized by absorption spectroscopy and the kinetic curves. Results show thatdifferent photochemical reactions are responsible for the reduction of Au (Ⅲ) at different pH values. The transmission electron microscopy (TEM)images indicate that when the pH value of the solution equals to 2.4 (higher citric acid concentration), 4, 5, and 12, respectively, the increase ofcitric acid concentration evidently leads to the decrease of particle size as well as σ value, except for 9.5mM (pH 5). However, Au nanoparticlesexhibit slight growth in size as the citric acid concentration increases from 0.95 to 3.8 mM, when the pH value of the solution equals to 7 and 10,respectively. While, on the whole, the increase of pH value could reduce the particle size and the σ value at the same citric acid concentration.However, both the mean diameter and σ value reach to minimum at pH 5. A UV induced aggregation, melting and fragmentation process informing gold nanoparticles are observed. An atom-by-atom dissolution mechanism is considered to be responsible for this process. Based on theseobservations, a modified model in formation of gold nanoparticles is discussed.