Abstract: The viscosity of the medium is one of the important factors for citric acid productivity in shake culture, so in this study the effect of various viscous substances on citric acid production by Yarrowia lipolytica DSM3286 and isolated wild type Yarrowia lipolytica M7 were investigated. The addition of 0.25 g L-1 agar as a viscous additive to the medium containing glucose as a carbon source increased slightly the medium viscosity but substantially increased the citric acid yield on shake culture in Y. lipolytica M7 and Y. lipolytica DSM3286 by 28 and 40%, respectively. Different morphologies of the DSM3286 strain were observed when cultivations were done in shake culture with or without the addition of agar. Also addition of 0.25 g L-1 carboxy methyl cellulose (CMC) had positive effect on citric acid production by Y. lipolytica strains however addition of gelatin and alginate had negative results. On the other hand PEG 4000 didn’t have significant effect on citric acid production. Since Y. lipolytica does not utilize these viscous substances, these results suggested that the viscous substances functioned as protectants for the yeasts from physiological stresses due to shaking and as a consequence resulted in a remarkably increased citric acid yield in shake culture.