The effects of various electropolishing (EP) parameters such as duration, applied voltage, working temperature, mechanical prepolishing and acid volume ratio on the EP process, and surface smoothness of high purity niobium (Nb) were systematically investigated using electrochemical technique and optical profilometry. The electropolishing parameters had a significant influence on the smoothness of the Nb by controlling the current density. Prolonged EP duration caused different dissolution rates of grains. Applied voltages above the oxygen evolution potential resulted in severe pitting of the Nb. High working temperature-caused roughening of the Nb surface via different dissolution rates of grains and mechanically ground smooth surfaces were roughened by the formation of valleys on the surfaces. A lower fluoride ion concentration in the acid solution gave rise to a rougher surface of the Nb because the fluoride ions were transport-limited species during the EP process.