Thermostable esterase gene was cloned (Est-AF) from extremophilic microorganisms, Archaeoglobus fulgidus DSM 4304. The protein analysis result showed that Est-AF is monomer with total 247 amino acids and molecular weight of estimated 27.5kDa. It also showed repeating units G-X-S-X-G (GHSLG) (residues $86{sim}90$) which is reported as active site of known esterases, and the putative catalytic triad composed of Ser88, Asp198 and His226. The esterase activity test with various acyl chain length of p-nitrophenol resulted that Est-AF showed highest specific activity with p-nitrophenylbutyrate $(pNPC_4)$ and rapidly decrease with pnitrophenyl ester contain more than 8 carbon chain. These results represent that cloned enzyme is verified as a carboxylesterase but not a lipase because esterase activity is decreased with p-nitrophenyl ester contains more than 8 carbon chains but lipase activity does not affected with carbon chain length. Optimum temperature of esterase reaction with p-nitrophenylbutyrate $(pNPC_4)$ was $80^{circ}C$. When ketoprofen ethyl ester was used as a substrate, activity of Est-AF showed the highest value at $70^{circ}C$, and 10% of activity still remains after 3 h of incubation at $90^{circ}C$. This result represents Est-AF has high thermostability with comparison of other esterases that have been reported. However, Est-AF showed low enantioselectivity with ketoprofen ethyl esterOptimum pH of Est-AF is between pH 7.0 and pH 8.0Km value of ketoprofen ethyl ester is 1.6mM and, $V_{max}$ is $1.7{mu}$mole/mg protein/min. Est-AF showed similar substrate affinity but slower reaction with ketoprofen ethyl ester compare with esterase from mesophilic strain P. fluorescens.