Doogh, an acidified milk drink made from yoghurt, water and salt, is a favorite refreshing beverage in Iran and some parts of Middle East. Phase separation of Doogh during shelf life, however, is believed to be the main obstacle in the way of its overall consumption. The problem is likely caused by large colloidal particles of doogh, low pH and the aggregation of casein. Considering the high polluting power of whey and the existing problems for its disposal as well as its high nutritional value, has made it essential to carry out a wide research on use of product in formulations and compounds different foods is done. In recent years, many attempts have been put in order to produce whey protein concentrate. So far, little research has been done on the stability of acidified milk drinks ??with whey proteins. On the other hand, pH and ionic strength of environment are the two effective factors in the stability of such products. The purpose of this study was to investigate the effects of whey proteins, pH and dissolved salt in milk, on the colloidal stability of Doogh. In order to fulfill these requirements five concentrations of whey protein from 0.5% to 4.0%; three pH levels 3.5, 4 and 4.5 and five concentrations (0%, 0.5%, 1%, 1.33% and 2%) of dissolved salt in the milk were used. The influence of the foregoing factors on the amount of phase separation, sedimentation and the product viscosity using the RSM design and during 28 days storage of product at 5°C were studied. Furthermore the micro structure, zeta potential, particle size distribution and sensory characteristics of the samples were investigated. It was found that appropriate models to determine the amount of phase separation and sedimentation were quadratic models, and 2FI for determining the viscosity. It was observed that with increasing amount of whey protein and salt in milk, phase separation reduced. While increasing whey protein and decreasing pH, decreased amount of sedimentation. Increasing pH and salt concentration in milk and whey proteins increased samples viscosity. In addition increasing the amount of whey protein in Doogh, reducing pH of product and the amount of salt dissolved in the milk reduced the size of the colloidal particles of Doogh. All of Doogh samples showed Newtonian behavior, but in samples with pH 4.5 and 4% whey proteins and 2% salt dissolved in milk was observed pseudo-plastic behavior. Samples with lowest phase separation had higher zeta potential value and zeta potential of samples with pH 3.5 and 4.5 were positive and negative, respectively. The results of Experiments during storage of the product showed, increasing the amount of whey proteins and salt not only reduced amount of phase separation of product during the storage but also the rate wass reduced. In all the samples viscosity of product during storage was constant, but in samples with pH 4.5, 4% whey proteins and 2% dissolved salt in milk viscosity decreases during storage. Investigating sensory properties of product showed the sample containing 0.5% whey protein, in terms of flavor and overall acceptability of product compared with the sample containing 4% whey protein has been more acceptability. Key Words : Doogh, Whey Proteins, pH, Salt, Colloidal Stability, Phase Separation, Sedimentation, Viscosity, RSM Design