Using nanofluids as absorbent is a novel technique for the enhancement of mass transfer in gas absorption process. In this article, a bubble column absorption system was used to investigate the effects of nanoparticle size on carbon dioxide (CO2) absorption in silica/water nanofluid, In the experimental setup, CO2 was injected as frequent bubbles from the bottom of the column containing silica/water nanofluid with different particle sizes of 10.6, 20, 38.6, and 62 nm. The rate of carbon dioxide absorption in silica-water nanofluid increased with nanoparticle size and maximum absorption rate or molar flux was obtained at 0.01 wt% of nanoparticles for all nanofluids with different nanoparticles sizes. Mass transfer parameters including diffusion coefficient, renewal surface rate and thickness of diffusion layer within liquid was also investigated at different nanoparticle sizes. The results show that by increment of nanoparticle size, the renewal surface rate increased while diffusivity and liquid-film thickness decreased. Consequently, Mass transfer coefficients increased by increment of nanoparticle size. Finally a new correlation was proposed incorporating nanoparticle diameter for estimating mass transfer coefficient.