The nucleation and initial growth of Si films is fundamental to the understanding and control of rapid thermal chemical vapor deposition (RTCVD), and plasma enhanced CVD(PECVD) processes. Herein is reported the nucleation and growth of Si deposited on oxidized silicon wafers by electron cyclotron resonance (ECR) PECVD in the 600–700 °C temperature range, under low pressures, and using both in situ spectroscopic ellipsometry and single wavelength real-time ellipsometry. An island growth and coalescence model is used to interpret the real-time ellipsometry data. Initial nuclei distance and growth parameters are derived. Atomic force microscopy was used to observe the early stage of nucleation. In situ spectroscopic ellipsometry is used to measure the final Si film thicknesses and optical properties. The deposited Si films were characterized by cross-sectional transmission electron microscopy.