In crystal plasticity models the crystal anisotropy of the yield strength is accounted for by the yield locus. In the present paper the full constraint Taylor model is used to calculate the yield strength anisotropy of a heavily cold rolled and annealed IF steel. In addition to the crystallographically induced anisotropy also the grain shape anisotropy was taken into account. To this purpose a model is presented in which the grain size that appears in the Hall-Petch relation is substituted by an effective grain size that is dependent of the grain-shape morphology and the crystal orientation. The grain shape of a specific crystal orientation is approximated by an ellipsoid volume of which the major axes are obtained from experimental data. The effective grain size of a specific crystal orientation is determined by the intersection of the most active crystal slip plane of this orientation and the ellipsoid volume.