Phenols constitute a widespread class of water pollutants that are generated from many industries and are known to cause a significant threat to the aquatic environment. Phenols are, therefore, considered as dangerous pollutants by global international quality organizations. This has led to a growing demand for an efficient technology for phenol removal from wastewater. Different sizes of amino-modified silica nanoparticles (SiNPs) were synthesized with 10-40nm in diameter (AMS-10 to 40), and their properties were characterized in terms of size and surface modification using transmission electron microscope (TEM), dynamic light scattering (DLS), zeta potential, elemental analyses (C, H, N), thermal gravimetric analysis (TGA) and Fourier transform infra-red (FTIR). The adsorption process was carried out utilizing batch mode experiment; the influence of various factors including pH of the medium, the contact time, the initial concentration of the adsorbate and the dose of the adsorbent on the phenol adsorption efficiency of SiNPs of various sizes were investigated. Phenol removal efficiency was found to be size-dependent, such that the phenol adsorption capacity of the SiNPs was in the following order: AMS-10>AMS-20>AMS-30>AMS-40 nm. The adsorption capacity and binding coefficient were calculated to be 35.2mg/g and 0.192mg/L, respectively, for AMS-10. The amino-modified SiNPs were found to be promising adsorbents for the phenol ions removal from the aqueous medium.