Disclosed are a novel gene coding for L-arabinose isomease derived from Thermotoga neapolitana 5068, a thermostable arabinose isomerase expressed from the said gene, a recombinant expression vector containing the said gene, a microorganism transformed with the said expression vector, a process for preparing thermostable arabinose isomerase from the said transformant and a process for preparing D-tagatose employing the said enzyme. Since the recombinant arabinose isomerase is highly thermostable and can produce tagatose with high yield at high temperature, it can be efficiently applied in pharmaceutical and food industries.