The present invention relates to a process for preparing a catalyst component for polymerization of an olefin comprising the steps of: i) contacting a compound R9zMgX2-z wherein R9 is aromatic, aliphatic or cyclo-aliphatic group containing up 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 given a first intermediate reaction product; ii) contacting the first intermediate reaction product with at least one activating compound selected from the group formed by electron donors and compounds of formula M(OR10)v-w(OR11)w, wherein M can be Ti, Zr, Hf, Al or Si, and M(OR10)v-w(R11)w, wherein M is Si, each R10 and R11, independently, represent an alkyl, alkenyl or aryl group, v is the valency of M, v being either 3 or 4, and w is smaller than v, to give a second intermediate reaction product; and iii) contacting the second intermediate reaction product with a halogen-containing Ti-compound, a monoester as activating agent, a 1,3-diether as an internal electron donor, and optionally a diester as an additional internal electron donor.