The Al-Mg-Mn alloy sheets with and without trace Sc and Zr were investigated by means of tensile test, X-ray diffraction, optical microscope, and transmission electron microscope. The indexes of in-plane anisotropy (IIPA) of their tensile mechanical properties were calculated and their inverse pole figures were obtained by Harris method. The two alloy sheets have the same law of in-plane anisotropy and remarkable in-plane anisotropy of mechanical properties, and the IIPA of the alloy sheet with Sc and Zr is bigger than that of the alloy sheet without Sc and Zr. The relationships of the in-plane anisotropy and the anisotropy of the crystallographic texture were analyzed based on the model of monocrystal. It is the common action of the anisotropy of crystallography and microstructures that causes the in-plane anisotropy of their mechanical sotropy of the alloy sheet containing trace Sc and Zr.