Nickel-doped tin oxide nanoparticles (sub-5nm size) with intense fluorescence emission behavior have been synthesized by sol–gel route. The structural and compositional analysis has been carried out by using XRD, TEM, FESEM and EDAX. The optical absorbance spectra indicate a band gap narrowing effect and it was found to increase with the increase in nickel concentration. The band gap narrowing at low dopant concentration (<5%) can be assigned to SnO 2 −SnO 2−x alloying effect and for higher doping it may be due to the formation of defect sub-bands below the conduction band.