beta-N-acetyl-d-glucosaminidase (NAGase, EC.3.2.1.52), a composition of the chitinases, catalyzes the cleavage of N-acetylglucosamine polymers into N-acetylglucosamine. In this paper, the effects of mercuric ion on the activity of NAGase from Penaeus vannamei for the hydrolysis of pNP-NAG have been studied. The results show that HgCl2 can lead to irreversible inactivation to this enzyme. The inactivation process follows a first-order reaction and the inactivation rate constants have been determined. The relationship between the inactivation rate constants and HgCl2 concentration has been studied and the result shows that only one molecule of HgCl2 binds to the enzyme molecule to lead the enzyme lose its activity. Moreover, the conformational changes of the enzyme inactivated by HgCl2 were studied by following changes in the intrinsic fluorescence emission and ultraviolet absorption spectra.