Band gaps are one of the most important parameters of semiconductor materials for optoelectronic applications since they determine the spectral features of absorptions and emission processes.In this paper, I will report our recent progress on the band gap engineering of semiconductor nanowires.Using some examples, I will show how to realize nanowire band gap tunability through composition control, and how to achieve graded bandgap design based on a single chip and along a single wire[1],respectively.The engineered nanowire alloys can give continuously tunable gaps (light emissions) covering the entire ultraviolet-visible wavelength range.