Niobium films with constant thickness have been deposited on sapphire (11 $($) over bar$$ 20) by electron-beam evaporation at different substrate; temperatures (150 degrees C less than or equal to T-S less than or equal to 750 degrees C). The samples were characterized by X-ray diffraction and resistivity measurements. X-ray reflectivity shows that all films are covered with an oxide layer of about 20 Angstrom in ambient atmosphere. The (110) texture at high T-S decreases towards lower T-S, accompanied by an increasing surface roughness. Below T-S = 350 degrees C the grain size in the growth direction becomes smaller than the film thickness and a relaxation of intrinsic stress is observed. A transition from a columnar growth structure to a fine grained microstructure is inferred. The change of the microstructure with T-S is probably due to the temperature dependent grain boundary mobility during the deposition process.