For the first time, an ultrathin iron-porphyrin based polymer nanocapsule with multiple peroxidase-like catalytic centers was constructed by covalently assembling iron-porphyrin monomers; this nanocapsule with a single molecule thickness shell acted as a highly efficient artificial enzyme for mimicking peroxidase. On the basis of the peroxidase-like activity of Fe-TPyP based nanocapsules (Fe-TPyP NCs), a highly sensitive colorimetric sensor for glucose determination was fabricated, the limit of detection was found to be as low as 0.098 μM. This study provided a novel strategy for developing artificial enzymes based on covalently assembled nanostructures. Furthermore, the colorimetric sensor for glucose determination showed potential applications in biomedicine and biology.