In this study, mesoporous silica nanoparticles (MSN) with different pore diameters were used for encapsulation of gallic acid (GA). All nanoparticles were characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, N₂ adsorption isotherms, scanning electron microscopy, X‐ray diffraction, and z‐potential analysis for determination of their characterizations. The effects of different pore sizes of nanoparticles on loading of GA and its release from GA‐loaded nanoparticles into simulated gastric fluid (SGF, pH 1.2), simulated intestinal and colon fluids (SIF and SCF, pHs 6.8 and 7.4, respectively), simulated body fluid (SBF, pH 7.4), and simulated gastrointestinal tract fluid (SGITF) were investigated. The pore size increased with the increase of mixing time of mesitylene as a swelling agent. GA is an unstable molecule that decomposes in alkaline pH. The imidazole buffer is the best for investigation of GA release into the alkaline media. The increase of nanoparticles pore size made the enhancement of GA loading. The released GA from GA‐loaded nanoparticles into SGF was slower than that into SIF and SBF. Release of GA was strongly pH dependent on the selected media. The antioxidant activity, based on the 1,1‐diphenyl‐2‐picrylhydrazyl assay, of released GA into SGITF and SBF was conserved and correlated strongly with its content in the selected media. © 2014 Curtin University of Technology and John Wiley & Sons, Ltd.