In order to develop gold nanoparticles that effectively accelerate the longitudinal relaxation of water protons, we have prepared gold nanoparticles coated with a monolayer of manganese(III)-porphyrin that can form four Au-S bonds on the gold surface. The particle size can be easily controlled via the initial S/Au ratio before reduction. The longitudinal relaxivity per Mn clearly increased with the particle size due to the rigid anchoring of manganese(BI)-porphyrin via four Au-S bonds.