ZrO 2 thin films in thickness of 17 nm to 559 nm were deposited on Si-(1 0 0) and glass substrates using mid-frequency reactive magnetron sputtering method. The microstructure analysis showed that the stable monoclinic phase preferentially grew in the initial growth stage, whereas the relative amount of tetragonal phase increased steadily with the increase of film thickness beyond the critical threshold (about 125 nm in the present work). The different evolution of phase component with that in the case of chemical vapor deposition is considered due to the additional impact of energetic particles in the sputtering process. The film thickness also showed significant impacts on residual stress, refractive index and transmittance of the films. The stress magnitude changed remarkably during the early stages of film growth, and then tended to a stable compressive value. The optical transmittance degraded gradually in the initial growth stage, meanwhile the refractive index increased progressively until the bulk value was reached. The fact that both microstructural and optical properties of ZrO 2 films are sensitive at thinner thickness indicates that the technical control of the sputtering process in the initial growth stage is crucial for microstructural and functional optimization of ultra-thin films.