  We report on a systematic study of strain effects on the transport properties
and the ferromagnetic transition temperature $T_{c}$ of high-quality
La$_{0.9}$Sr$_{0.1}$MnO$_{3}$ thin films epitaxially grown on (100) SrTiO$_{3}$
substrates. Both the magnetization and the resistivity are critically dependent
on the film thickness. $T_{c}$ is enhanced with decreasing the film thickness
due to the compressive stain produced by lattice mismatch. The resistivity
above 165 K of the films with various thicknesses is consistent with small
polaronic hopping conductivity. The polaronic formation energy $E_{P}$ is
reduced with the decrease of film thickness. We found that the strain
dependence of $T_{c}$ mainly results from the strain-induced electron-phonon
coupling. The strain effect on $E_{P}$ is in good agreement with the
theoretical predictions.
