This study investigates the synthesis of mesoporous SiO₂ nanoparticles with uniform spherical morphology using the sol-gel method. One of the strategies employed in this research is the control of particle size within the sol, achieved by adding ammonium polycarboxylate (APC). Various characterization techniques, including DLS, XRD, FTIR, DTA/TG, and SEM, were used to evaluate the product formation mechanism in the sol-gel process. It was demonstrated that by maintaining the pH at an acidic level of 3, precursor particles containing the Si source formed within the sol at sizes below 10 nm. This phenomenon is crucial for controlling the final product size to be under 50 nm. The addition of APC stabilized the precursor particles within the sol through an electrosteric mechanism. Scanning electron microscopy (SEM) images revealed that the silica nanoparticles had a spherical shape and a uniform size of approximately 25 nm. N₂ adsorption/desorption porosimetry indicated that the nanoparticles possessed mesopores with an average pore size of 2.5 nm and a specific surface area of 120 m²/g. This method can be easily utilized to produce silica nanoparticles with desirable morphology and mesostructure