Nanoscale particles of rutile- and anatase-type titanium dioxides were selectively grown in acidic solutions of titanyl sulfate. The variation of photocatalytic activity as a function of UV light intensity was investigated using rutile and anatase powders having almost the same size (20-30 nm) and shape for a clear comparison of the performance depending on the crystal structure. Whereas the performance of anatase evaluated with the decomposition of methylene blue and the reduction of Cr(VI) under UV light was proportional to the square root of the UV light intensity, rutile showed a saturation of the activity in the high-intensity region regardless of the light sources and photochemical reactions. The saturating tendency of rutile weakened with an electron-withdrawing reaction or a decrease in the grain size. Then, the performance of rutile could be superior to that of anatase under illumination at a low-light-intensity. The difference in the photocatalytic performance between rutile and anatase is tentatively ascribed to a high recombination rate of generated electrons and holes associated with a low mobility of carriers in rutile.