We performed systematic studies on the transport properties of FeSe thin films with controlled degrees of in-plane lattice strain, including both tensile and compressive strains. The superconducting transition temperature, $T_{mathrm c}$, increases up to 12 K for films with compressive strain while the superconductivity disappears for films with large tensile strains. On the other hand, the structural (nematic) transition temperature, $T_{mathrm s}$, slightly decreases as the in-plane strain is more compressive. This suggests that the structural transition can be extinguished by a smaller amount of Te substitution for films with more compressive strain, which may lead to higher $T_{mathrm c}$ in FeSe$_{1-x}$Te$_x$. It was also found that the carrier densities evaluated via transport properties increase as the in-plane strain becomes more compressive. A clear correlation between $T_{mathrm c}$ and the carrier densities suggests that it is essential to increase carrier densities for the $T_{mathrm c}$ enhancement of iron chalcogenides.
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