Coexistence effect of titanate nanosheets (TNS) with nanometric lateral dimensions (ca. 3 nm), which were prepared through a hydrolysis reaction of titanium tetraisopropoxide, on catalytic activity of horseradish peroxidase (HRP) was investigated as a function of solution pH. Especially in diluted HRP solutions with a pH range of 7 ~ 8, enzymatic reaction rate, i.e., maximum velocity (Vmax) in the cconventional Michaelis-Menten equation, was significantly enhanced more than 2 times in the presence of TNS. In contrast, the zincrease in Vmax was not very large in acidic (pH = 4.0) and basic solutions (pH = 9.0). It was demonstrated that the TNS brought about peptization of aggregates composed of several HRP molecules in a diluted solution, causing increase in an apparent HRP concentration participating in the enzymatic reaction. Moreover, the coexistent TNS activated superoxide dismutase (SOD) with O2 -scavenging performance.