C-axis oriented La0.7Sr0.3MnO3−δ (LSMO) films were fabricated on the top of SrTiO3/YBa2Cu3O7 grown on MgO (001) substrates. From x-ray ϕ-scan and planar transmission electron microscopy measurements, the LSMO layer in the LSMO/SrTiO3/YBa2Cu3O7/MgO heterostructure is found to have coherent in-plane grain boundaries with a predominance of 45° rotations (between  and  grains) in addition to the cube-on-cube epitaxial relationship. Also, epitaxial LSMO/Bi4Ti3O12/LaAlO3 (001) and c-axis textured LSMO/Bi4Ti3O12/SiO2/Si (001) with random in-plane grain boundaries are introduced as the counterparts for comparison. The resistivity and magnetoresistance (MR) of LSMO layer were measured and compared. The low field MR at low temperature shows a dramatic dependence on the nature of the grain boundary. An attempt is made to interpret these results on the basis of correlation between the magnetic properties and grain structures. © 1998 American Institute of Physics.