The adsorption behavior and enzyme activity of horseradish peroxidase (HRP) was examined on a synthetic clay nanosheet, whose surface is flat at the atomic level and is negatively charged. The results showed that HRP is adsorbed effectively (adsorption equilibrium constant, K = 1.61 X 10(7) L mol(-1)) and that the structure of HRP was altered on the clay surface. The enzyme activity of HRP on the clay surface was evaluated by using H2O2 and tert-BuOOH as a substrate. As a result, HRP on the clay surface was able to work for tert-BuOOH, while HRP in solution did not show any activity. In addition, HRP on SSA showed reactivity even under the high-temperature conditions. These results indicate that the clay nanosheet can be a unique modifier for enzyme activity of HRP.