Some species of lactic acid bacteria produce exocellular polysaccharides (EPS). EPS play major technological role in the production of fermented dairy products, particularly production of yoghurt. Their production considerably contributes to texture, rheology, mouthfeel, taste perception and stability of the final products. In addition, health benefits have been attributed to some of the bacterial EPS, particularly, antitumor and immunomodulating activities. The aim of this study was to determine EPS concentration in some of traditional and commercial yoghurts and to investigate EPS effects on physical characteristics of these products. Three traditional yoghurt (S1, S2 made of mixture of cow and sheep milk, and G: made of cow milk) were compared with a widely distributed: industrial yoghurt type (I). Experimental data showed that there were significant differences in chemical and physical properties of the yoghurt samples including fat content, solid-non fat content , pH, EPS concentration, apparent viscosity, extensibility and susceptibility to syneresis. Statiscal analysis showed that amount of EPS and viscosity or decreasing susceptibility to syneresis were highly correlated (p 0.05) where as no direct correlation were found between SNF or fat content and physical properties of above mentioned samples. To eliminate effect of milk composition on the physico-chemical properties of yoghurt, reconstituted skim milk used to prepare yoghurt using four yoghurt samples as the starter at 37°C and 42°C. Result showed that physical properties of yoghurt made by different mix culture were significantly different. As expected, in general 37°C seemed to produce better quality yoghurt than 42°C. Bacterial mixture in S2 sample produced higher amount of EPS and at the same time highest viscosity among other samples, therefore extensibility and lower syneresis were observed in the yoghurt made by it. The results suggest that the physical characteristics of yoghurt can be improved by the use of EPS-producing starter culture. Moreover, the Iranian traditional yoghurts are the important sources for isolation and production of these starters for future trends.