The changes in the activity and the conformation of the hyperthermophilic esterase derived from aerobic thermophilic Aeropyrum pernix K1(APE1547) were studied during denaturation by guanidine hydrochloride(GdnHCl) and urea. The denaturation course of APE1547 was followed by the steady-state and time resolved fluorescence methods. An increase in the denaturant concentration in the denatured system can significantly enhance the inactivation and unfolding of APE1547. The enzyme can be completely inactivated with a urea concentration of 2.7 mol/L or a GdnHCl concentration of 7.5 mol/L. The fluorescence emission maximum of the enzyme protein red shifts in magnitude to a maximum value(355 nm) when the concentration of GdnHCl is 5.1 mol/L. The experimental results indicate that APE1547 has a high resistance to urea. Unfolding of APE1547 in GdnHCl(4.2—6.0 mol/L) was shown to be an irreversible process. The present results indicate that the ion pairs in this protein may be a key factor for the stability of this esterase. The changes in the activity and the conformation of the hyperthermophilic esterase derived from aerobic thermophilic Aeropyrum pernix K1 (APE1547) were studied during denaturation by guanidine hydrochloride (GdnHCl) and urea. The denaturation course of APE1547 was followed by the steady-state and time resolved fluorescence methods. An increase in the denaturant concentration in the denatured system can significantly enhance the inactivation and unfolding of APE1547. The enzyme can be completely inactivated with a urea concentration of 2.7 mol / L or a GdnHCl concentration of 7.5 mol / L. The maximum concentration of the enzyme protein red shifts in a maximum value (355 nm) when the concentration of GdnHCl is 5.1 mol / L. The experimental results indicate that APE1547 has a high resistance to urea. Unfolding of APE1547 in GdnHCl (4.2- 6.0 mol / L) was shown to be an irreversible process. The present results that that ion pairs in this protein may be a key factor for th e stability of this esterase.