l-Ribose is a synthetic l-form monosaccharide. It is a building block of many novel nucleotide analog anti-viral drugs. Bio-production of l-ribose relies on a two-step reaction: (i) conversion of l-arabinose to l-ribulose by the catalytic action of l-arabinose isomerase (l-AI) and (ii) conversion of l-ribulose to l-ribose by the catalytic action of l-ribose isomerase (l-RI, EC 5.3.1.B3) or mannose-6-phosphate isomerase (MPI, EC 5.3.1.8, alternately named as phosphomannose isomerase). Between the two enzymes, l-RI is a rare enzyme that was discovered in 1996 by Professor Izumori's group, whereas MPI is an essential enzyme in metabolic pathways in humans and microorganisms. Recent studies have focused on their potentials for industrial production of l-ribose. This review summarizes the applications of l-RI and MPI for l-ribose production.