The aim of this study was to evaluate the quality attributes of yoghurt made from cow's milk (3.6%fat and 3.37% protein) fortified with different levels of milk protein (0.5, 1.0 or 1.5%) as skim milk powder (SMP), sodium caseinate (NaCn) and dried whey protein concentrate (WPC); or ultrafiltrated (UF) to similar protein levels.                    The results indicated that titratable acidity (TA),  acetaldehyde (AC) and diacetyl (DA) contents as well as the firmness (Fr), as inversely indicated from the penetration value, consistency coefficient (CC) and yield stress (YS) of yoghurt raised as the protein fortification level increased, by which the reduction in pH values were delayed. SMP-fortified yoghurt had the highest contents of TA, AC and DA and the lowest pH value, followed by WPC, UF and NaCn, which caused the greatest Fr and YS followed by UF. Proportional increases in TA and all rheological parameters were determined, while pH value decreased continuously until the end of SP. The increment in the AC and DA contents retreated after the 7^th and 14^th day respectively. Gradual increment in Streptococcus. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus counts associated with protein elevation level was determined. SMP in yoghurt promoted the highest viability conditions for both strains followed by WPC, UF and NaCn. The duration of storage period (SP) for 21 days led to gradual reduction in counts of both strains. In conclusion, the yoghurt with increased 1.0% protein by UF, attained the highest total sensory scores, and was kept with stable sensory quality until the 14^th day and stilled acceptable until the end of storage period.