Bacillus subtilis alpha-amylase, which contains a relatively large amount of alpha-helix, was adsorbed on two types of ultrafine silica particles (silica-1 and -2, average diameter 15 nm) under various conditions. The changes in circular dichroism (CD) spectra of alpha-amylase upon adsorption were measured, and the extent of conformational changes was estimated from the reduction in alpha-helix content. In addition, the activities of adsorbed alpha-amylase were measured at pH 5.2 using corn starch and p-nitrophenylbenzyl alpha-maltopentaoside (BG5P). In the ultrafine silica-2 particles, the extent of both activity reductions and conformational changes upon adsorption was much larger than that in the ultrafine silica-1 particles and increased with decreasing pH and amount of adsorption. The extent of activity reductions correlated closely with the conformational changes. On the other hand, the effect of reduction in alpha-amylase activity upon adsorption measured by BG5P was smaller than that measured by starch, indicating that the lack of accessibility of the active site to a large substrate also reduces the activity of adsorbed alpha-amylase. However, the effects of particle type and adsorption conditions on the extent of activity reductions by the accessibility resistance were small. Therefore, variation of the activity of adsorbed alpha-amylase is mainly attributable to the extent of conformational changes upon adsorption. Based on these results, a procedure to prepare adsorbed alpha-amylase with high activity was investigated.