The mutant strains of Streptococcus thermophilus and Lactobacillus delbruekii subsp. bulgaricus were used to improve of yoghurt quality. The produced yoghurts were kept refrigerated and analyzed when fresh, 7, 14, 21 and 28 days for chemical, microbiological and sensory characteristics. The obtained results revealed that the prolonging of the coagulation time of some treatments to be with a maximum of 7.20 and 6.20 hr for T3 and T6, respectively. No pronounced differences were observed all over the storage period in the chemical composition of yoghurt including T.S, Fat and protein content, T2 and T5 recorded the highest content of acetaldehyde. Moreover, using of the mutant of starter cultures of Streptococcus thermophilus and Lactobacillus delbruekii Subsp. bulgaricus improved the organoleptic properties, mild taste and the shelf life of the produced yoghurt compared with the control.