We investigated how the crystallinity of ZnO host films deposited on Si(100) substrates by electron cyclotron resonance (ECR) plasma sputtering affects the optical activity of Er 3+ ions doped in them under resonant excitation with 532-nm laser light. Photoluminescence from ZnO:Er films deposited at room temperature (RT) exhibited a main emission peak at 1538nm with a sub-peak at 1553nm. The emission intensity reached a steady state at Er content between 0.6 and 3at.%, and attenuated above 3at.%. This concentration quenching correlates with derivation of X-ray diffraction angles from the Vegard's law. Deposition at RT produced non-equilibrium state of ZnO:Er films possibly stabilized with vacancies to maintain charge neutrality. After post annealing of ZnO:Er films with Er content less than 1at.%, the emission intensities were markedly reduced as the crystalline lattice was rearranged. For films in the concentration quenching range, however, post annealing increased the emission peak width while maintaining emission intensity, indicating that Er 3+ ions migrated and populated various emission-active sites. In this case, the main peak wavelength blue shifted to 1536nm and produced a new sub peak at 1546nm. When the deposition temperature exceeded 500°C, emissions from Er 3+ ions were greatly enhanced. This will result from improvement in crystallinity and fixing Er 3+ ions at six-fold coordinated C 4v symmetry sites.