Lactosucrose ($4^G-eta$-D-galactosylsucrose) is an oligosaccharide consisting of galactose, glucose, and fructose. In this study, we prepared lactosucrose from lactose and sucrose using a levansucrase derived from Zymomonas mobilis. Optimum conditions for lactosucrose formation were $23^{circ}C$, pH 7.0, 18.0% (w/v) lactose monohydrate, and 18% (w/v) sucrose as substrates, and 1 unit of enzyme/ml of reaction mixture. Under these conditions, the lactosucrose conversion efficiency was 28.5%. The product was purified and confirmed to be O-$eta$-D-galactopyranosyl-($1{ ightarrow}4$)-O-$eta$)-D-glucopyranosyl-($1{ ightarrow}2$)-$eta$-D-fructofuranoside, or lactosucrose. A mixed-enzyme system containing a levansucrase and a glucose oxidase was applied in order to increase the efficiency of lactose and sucrose conversion to lactosucrose, which rose to 43.2% as a result.