Abstract: In the present investigation, the effect of base oil viscosity and fume silica concentration on properties of formulated greases has been investigated. Mixture of greases is formulated using poly-methylsiloxane (with viscosity 500, 1000 and 2000 cS at 25°C) as base oil and thickened with 5-13% wt. fume silica. The results of testing have been done on formulated grease (penetration test, oil separation test, drop point test, corrosive substance, stability of the consistency at high temperature test) show that the increase in the fume silica concentration at same base oil viscosity increases the consistency of formulated grease, while the oil separation rapidly decreases by increasing the fume silica concentration at same oil viscosity. After 10% wt. addition of fume silica, oil separation becomes approximately unchanged. The result of the study of the effect of the base oil viscosity on the oil separation and consistency of formulated grease using 10% addition of fume silica shows that oil separation decreases and consistency increase with viscosity. No drop point of formulated greases was discovered (340°C+) and this means that the structure of formulated grease remains stable.