Effect of fa contet of milk (2.7%, 5%, 7% and 9%) on the growth and metabolic activity of starter cultures during fermentation of concentrated yoghurt (23% total solid), was studied.Sensory assessment of yoghurt samples were also obtained. The characteristics that was measured during fermentation include: PH, acidity, count of starter bacteria (stereptococcus thermophilus and Lactobacillus bulgaricus) , and the characteristics that measured in final product were sensory properties.The experiments were done in three replications . Statistical evaluation showed that increase in fat content of milk, had no significant effect on PH but resulted decreasing of acidity of samples. Addition of fat content from 2.7% upto 9% decreased acidity from 0.97% to 0.78%.with increasing in fat content from 2.7% upto 9% count of stereptococcus thermophilus decreased from 391×106 cfu/ml to 279×106 cfu/ml where fat content had no significant effect on growth of Lactobacillus bulgaricus. Addition of fat content did not influenced the sensory acceptability of final product.