We report a facile route to synthesize size tunable Fe3O4 nanoparticles (NPs)-carbon nitride nanotube (CNNT) hybrids. These hybrids showing the water-soluble property are proven to exhibit ultra high peroxidase mimetic activity compared to those of pure NPs, where a colorless peroxidase substrate 3,3,5,5-tetramethyl-benzidine changes by H2O2 to its blue colored oxidized state.