Shape selectivity is one of the crown jewels of catalysis. It is frequently reported on microporous materials, mostly zeolite materials in gas phase reactions, but is rarely reported on mesoporous catalysts in liquid solution. Here we report reactant shape selectivity inside the mesoporous pores of Pd@meso-SiO2 nanoreactor, which is a hollow composite with mesoporous silica walls and Pd nanoparticles residing at the inside pore mouths. The composite has structural features that resemble a nanoreactor and shows extremely high activity for Suzuki cross-coupling reaction between phenylboronic acid and iodobenzene. We observed sharp shape selectivity on phenylboronic acid. N-Butyl phenylboronic acid resulted in an 86% product yield, while tert-butyl phenylboronic acid resulted in a 0% product yield. We propose that nanoreactor feature of the catalyst and collective diffusion barrier induced by the preferential adsorption of bulky phenylboronic acid molecules account for these observations.