We propose to determine the magnetic structure of the noncentrosymmetric heavy-fermion superconductor CePt3Si using polarized elastic neutron scattering on IN20 in zero field. IN20 is needed in order to separate the weak magnetic Bragg peaks from the strong inelastic scattering. If the present indications that the moment may be tilted out of the tetragonal basal plane are confirmed, this would have important implications for the understanding of both the unconventional superconductivity related to the Rashba-type spin-orbit interactions and the highly unusual magnetic field dependence of the magnitude of the ordered moment.