Kluyveromyces lactis β-galactosidase was immobilized on silica gels using a covalent bonding method. To improve lactulose synthesis using immobilized β-galactosidase, the optimal reaction conditions, such as lactose and fructose concentrations, pH and ionic strength of the buffer, loading amount of the enzyme and temperature, were determined. Lactulose synthesis using the immobilized β-galactosidase was markedly improved after optimization of the reaction conditions. When the lactulose synthesis was carried out at 47 ℃ using 40% (w/v) lactose, 20% (w/v) fructose and immobilized β-galactosidase of 12 U/ml in 50 mM sodium phosphate buffer at pH 7.5, the lactulose concentration and specific productivity were 15.80 g/l and 1.32 mg/U·h, respectively. In addition, when the immobilized β-galactosidase was reused for lactulose synthesis, its catalytic activity retained 60.5% after 10 reuses.