Even though doping small sized NCs is a challenging task, 2.7 nm sized tin oxide (SnO 2 ) nanocrystals (NCs) are successfully doped with cobalt (Co). Studies are carried out in dilutely Co-doped SnO 2 NCs, so as to avoid Co cluster formation and interference due to extrinsic effect. Co 2+ ions are substitutionally doped in SnO 2 lattice as seen from the d - d transitions in optical absorption spectra. Further x-ray photoelectron spectroscopic measurements also confirm Co 2+ ionic state in SnO 2 NCs. In view of the known fact that ferromagnetism in NCs arises due to different defects, Raman measurements are carried out. Raman spectra reveal presence of “in-plane” oxygen vacancies, which is also substantiated by photoluminescence spectra. Even though, Co-doping is not profoundly affecting the structure of SnO 2 NCs, subtle ferromagnetic ordering observed in un-doped NCs get destroyed in Co-doped NCs. The present work reiterates the importance of Co-Co anti-ferromagnetic interaction and also indirectly supports the conjecture that oxygen vacancies adjacent with Co ions in SnO 2 lattice in a necessary condition for realizing ferromagnetic ordering.