A process for preparing a catalyst composition for polymerization of olefins, said process comprising the steps of: i) contacting a compound R 9 z MgX 2-z wherein R 9 is aromatic, aliphatic or cyclo-aliphatic group containing 1 to 20 carbon atoms, X is a halide, and z is in a range of larger than 0 and smaller than 2, with an alkoxy- or aryloxy-containing silane compound to give a first intermediate reaction product; ii) contacting the solid Mg(OR 1) x X 2-x with at least one activating compound selected from the group formed by compounds of formula M(OR 10) v-w (OR 11) w, wherein M is Si; and each of R 10 and R 11, independently, represent an alkyl, alkenyl or aryl group and wherein the groups differ independently, v is the valency of M, v being either 3 or 4 and w is smaller than v; and iii) contacting the second intermediate reaction product with a halogen-containing Ticompound, an internal electron donor represented by formula, wherein: R 1, R 2, R 3, R 4, R 5 and R 6 are the same or different and are independently selected from a group consisting of hydrogen, straight, branched and cyclic alkyl and aromatic substituted and unsubstituted hydrocarbyl having 1 to 20 carbon atoms; R 7 is selected from a group consisting of straight, branched and cyclic alkyl and aromatic substituted and unsubstituted hydrocarbyl having 1 to 20 carbon atoms; and R 8 is selected from a group consisting of aromatic substituted and unsubstituted hydrocarbyl having 6 to 20 carbon atoms; N is nitrogen atom; O is oxygen atom; and C is carbon atom. i) contacting a compound R 9 z MgX 2-z wherein R 9 is aromatic, aliphatic or cyclo-aliphatic group containing 1 to 20 carbon atoms, X is a halide, and z is in a range of larger than 0 and smaller than 2, with an alkoxy- or aryloxy-containing silane compound to give a first intermediate reaction product; ii) contacting the solid Mg(OR 1) x X 2-x with at least one activating compound selected from the group formed by compounds of formula M(OR 10) v-w (OR 11) w, wherein M is Si; and each of R 10 and R 11, independently, represent an alkyl, alkenyl or aryl group and wherein the groups differ independently, v is the valency of M, v being either 3 or 4 and w is smaller than v; and iii) contacting the second intermediate reaction product with a halogen-containing Ticompound, an internal electron donor represented by formula, wherein: R 1, R 2, R 3, R 4, R 5 and R 6 are the same or different and are independently selected from a group consisting of hydrogen, straight, branched and cyclic alkyl and aromatic substituted and unsubstituted hydrocarbyl having 1 to 20 carbon atoms; R 7 is selected from a group consisting of straight, branched and cyclic alkyl and aromatic substituted and unsubstituted hydrocarbyl having 1 to 20 carbon atoms; and R 8 is selected from a group consisting of aromatic substituted and unsubstituted hydrocarbyl having 6 to 20 carbon atoms; N is nitrogen atom; O is oxygen atom; and C is carbon atom. R 1, R 2, R 3, R 4, R 5 and R 6 are the same or different and are independently selected from a group consisting of hydrogen, straight, branched and cyclic alkyl and aromatic substituted and unsubstituted hydrocarbyl having 1 to 20 carbon atoms; R 7 is selected from a group consisting of straight, branched and cyclic alkyl and aromatic substituted and unsubstituted hydrocarbyl having 1 to 20 carbon atoms; and R 8 is selected from a group consisting of aromatic substituted and unsubstituted hydrocarbyl having 6 to 20 carbon atoms; N is nitrogen atom; O is oxygen atom; and C is carbon atom.