Germanium (Ge) films are prepared on BK7 glass and multispectral zinc sulfide (m-ZnS) substrates by ion beam assisted deposition (IBAD). The effects of substrate temperature, deposition rate, and ion energy on the microstructure and optical properties of the films are investigated. It can be concluded that Ge film deposited with higher rate or ion energy has more optical absorption, while ion energy below 150 eV helps to reduce film absorption. Film refractive index increases with film deposition rate and bombardment ion energy while it is below 300 eV. And higher growth rate or bombardment ion energy can weaken film diffraction intensity.