The present invention relates to a recombinant GRAS (Generally Recognized As Safe) strains expressing thermophilic arabinose isomerase as an active form and method of food grade tagatose by using the same, and more precisely, a gene encoding arabinose isomerase originating from the thermophilic Geobacillus stearothermophilus DSM22 and Geobacillus thermodenitrificans, a recombinant expression vector containing the gene, a recombinant GRAS strains expressing the thermophilic arabinose isomerase as an active form by transformed with the expression vector, and a method of preparing food grade tagatose from galactose by using the same.