The internal rotation of a photoexcited hemicyanine dye, trans-3-[4-(dimethylanlino)-styryl]-1-methylpyridinium iodide (DASPI), was investigated in water-containing reverse micelles. The rate constants for the twisting motion were obtained from the measured fluorescence lifetimes in various sizes of water nanoclusters (ca. 0.6-5.6 nm in radius) prepared from sodium bis(2-ethylhexyl) sulfosuccinate (AOT)-alkane-water solutions by changing the molar ratio of water/AOT. It was observed that the fluorescence lifetime of DASPI in the water pool formed by the reverse micelle markedly increases as the nanocluster size decreases, implying that the excited-state twisting motion is significantly influenced by the microstructure of the reverse micelle. An endeavor was made to relate the rate process for internal rotation to water pool characteristics such as polarity and viscosity.