The lattice deformation of dense strained La_0.7Sr_0.3MnO₃ (LSMO) films is shown to control the easy direction of the magnetization. Optimized pulsed laser deposited conditions allow the fabrication of dense LSMO thin films which present an exceptional flatness with a peak–valley roughness (Rp–v) of 1 Å, associated to epitaxial grains as large as 1 μm. Electron microscopy coupled with x-ray diffraction have been used to study the unit cell distortion of both tensile and compressive dense LSMO films as a function of the thickness. No relaxation of the lattice distortion imposed by substrate has been observed in the thickness range 10–60 nm. The Curie temperature is not significantly affected by the nature of the substrate: a TC of 350 K is observed for both SrTiO₃ (STO) and LaAlO₃ (LAO) substrates, i.e., close to the bulk material (369 K). In contrast, the easy direction of magnetization depends on the substrate. For tensile films deposited on the STO substrate, the unit cell is elongated along the film’s plane (ain-plane=3.905 Å) with a reduced perpendicular parameter (cperp=3.85 Å): an easy direction of magnetization M in the plane of the film is observed. For compressive films deposited on LAO substrate, the situation is reversed with a unit cell elongated along the direction of growth (cperp=4.00 Å and ain-plane=3.79 Å) and an easy axis for M along this perpendicular out-plane direction. It is thus demonstrated that the larger cell parameter, ain-plane for films deposited on STO and cperp for films deposited on LAO, is fully correlated to the direction of the easy magnetization.