Shear deactivation of cellulase and its major component enzymes, viz., exoglucanase (exo-1,4-beta -D-glucan-4-cellobiohydrolase), endoglucanase (endo-1,4-beta -D-glucanhydrolase), and 1,4-beta -glucosidase, was carried out by exposing cellulase to shear in a mechanically agitated reactor in the presence as well as in the absence of the substrate cellulose. Cellulase was found to undergo deactivation when subjected to shear, and the extent of deactivation increased with increasing speed of agitation. Among the three major component enzymes of cellulase, exoglucanase showed rapid deactivation and contributed the most to cellulase deactivation. The presence of a substrate did not affect the deactivation of cellulase.