Thin films of perovskite manganiteLa0.7Ca0.3MnO3were grown epitaxially onSrTiO3(100), MgO(100) andLaAlO3(100) substrates by the pulsed laser deposition method. Microscopic structures of these thin film samples as well as a bulk sample were fully determined by x-ray diffraction measurements. The unit cells of the three films have different shapes, i.e., contracted tetragonal, cubic, and elongated tetragonal forSrTiO3,MgO, andLaAlO3,respectively, while the unit cell of the bulk is cubic. It is found that the samples with a cubic unit cell show smaller peak magnetoresistance at low fields(≲1 T)than the noncubic ones do. The present result demonstrates that the magnetoresistance ofLa0.7Ca0.3MnO3at low fields can be controlled by lattice distortion via externally imposed strains. ©1997 American Institute of Physics.