The expression of sucrose isomerase(SIase)gene in recombinant E.coli BL21(DE3)induced by lactose and IPTG was investigated.We optimized the inducing conditions such as the culture temperature after induction,the point of induction,the inducer concentration and the induction time.The results showed that the optimal inducing conditions were to add 0.8 mmol/L(final concentration)IPTG(0.5 mmol/L lactose)after cell growth for 3 h,then incubate at 20℃(24℃ for lactose)for 14 h(12 h for lactose).After induction,the expression level of recombinant protein induced by lactose was about 27.2% of the total cellular protein,which was less than that induced by IPTG(41.6%).However,the SIase activity induced by lactose(14.72 U/mL)was higher than that induced by IPTG(12.37 U/mL),these results indicated the advantages of lactose as inducer in yield of SIase activity.Then sucrose with initial concentration of 500 g/L was converted to isomaltulose by the recombinant E.coli cells which was immobilized using sodium-alginat,the average isomaltulose yield and conversion rate for sucrose after 10~11 h was above 83% and 99% respectively.The activity of immobilized cells was stable after 25 batchs of conversion.The conversion efficiency was increased by nearly 55% compared with the original strain.