Monodispersed silica nanoparticles (SNPs) were synthesized by cost effective aqueous-ethanolic route. The particle size was controlled by optimizing the synthetic conditions. The synthesized different size SNPs were used to study the corrosion protection efficiency on aluminium (Al) in alkaline medium by weight loss, Potentiodynamic polarization and surface morphology examination methods. It was found that SNPs with smaller particle sizes showed better protection efficiency compared to those of larger particles. The results were explained in terms of effective surface coverage by smaller sized SNPs.