107 cfu/mL以上。 To improve the protective efficacy of Bifidobacterium longum BBMN68, various cryoprotectants were evaluated and optimized. Based on its viability and activity after frozen, the effects of eighteen cryoprotectants belonging to four different types were evaluated. Six of them including skim milk, trehalose, fructose, glycerol, Vitamin C and L-Glu were proved to be effective. Then the complex formulas made of the six kinds of the selected compounds was optimized by using a quadratic regression orthogonal experiment. And the optimized composition is as follows: skim milk 8%, trehalose 5%, fructose 5%, glycerol 3%, Vitamin C 0.05% and L-Glu 0.05%. With the protection of the optimized cryoptotectans, the stability of starters is good with a stable logarithm value about 10.5 lg(cfu/mL) during storage, and when applied in yoghurt fermentation it presents an excellent performance, with a total counts of culturable cells above 107cfu/mL during 21 days of storage.