The effects of guanidinium chloride (GuHCl) on the activity of Penaeus vannamei beta-N-acetyl-D-glucosaminidase (NAGase) have been studied. The results show that GuHCl, at appropriate concentrations, can lead to reversible inactivation of the enzyme, and the IC50 is estimated to be 0.6 M. Changes of activity and conformation of the enzyme in different concentrations of GuHCl have been studied by measuring the fluorescence spectra and its relative activity after denaturation. The fluorescence intensity of the enzyme decreases distinctly with increasing GuHCl concentrations, and the emission peaks appear red-shifted (from 339.4 to 360 nm). Changes in the conformation and catalytic activity of the enzyme are compared. The extent of inactivation is greater than that of conformational changes, indicating that the active site of the enzyme is more flexible than the whole enzyme molecule. The kinetics of inactivation has been studied using the kinetic method of the substrate reaction. The rate constants of inactivation have been determined. The value of k(+0) is larger than that of k'(+0) which suggests that the enzyme is protected by substrate to a certain extent during guanidine denaturation.