It could lay a foundation for the development of yogurt direct-vat-strain by analysising four Lactobacillus delbrueckii subsp bulgaricus strains' fermentation characteristics including acid-producing ability,postacidification property,texture and sensory quality of the yogurt which were fermented by single strain or combination with Streptococcus thermophilus strains.All of these strains were researched more in the KLDS.