Disclosed are a gene coding for L-arabinose isomerase derived from Thermotoga neapolitana 5068, a thermostable arabinose isomerase expressed from the gene, a recombinant expression vector containing the gene, a microorganism transformed with the expression vector, a process for preparing thermostable arabinose isomerase from the transformant ,and a process for preparing D-tagatose employing the arabinose isomerase. 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.