A multiplexed assay technique to measure the photocatalytic activity (PCA) of nanoparticles (NPs) in aqueous suspension was developed based on the observation of TiO(2) NPs-photocatalytic oxidation rate of NADH by monitoring the fluorescence intensities. 96 sample solutions of a small volume (<150 μL) could be assayed in a single run without separation of NPs within 15 min. PCA values can be measured with high sensitivity and low experimental uncertainties through the observation at various concentrations of photocatalyst, substrate, aqueous protons and pH buffer ions in a short measurement time.