Scanning tunneling microscopy (STM) was used to analyze the surface topography of rf sputtered Nb films, 10-1000 nm thick. Surface crystalline sizes within 100 nm have been observed. Grain sizes deduced from STM have been compared to those evaluated from x-ray diffractometry. The effect of both deposition rate and substrate temperature on surface roughness has also been studied on 100 nm thick Nb samples. All the results show a behavior typical of thin films grown under low mobility conditions, where a columnar structure prevails.