We present a study of the light emission properties over wavelengths from UV to blue of Mn doped ZnO nanocrystals fabricated by means of a thermal evaporation vapor phase deposition process. The samples were grown with a Mn mole ratio in the Zn/Mn mixed source of 0% (pure ZnO sample, used as a reference), 5%, 10%, or 15% in a constant O 2 /Ar gas mixture flowing at 500°C. The pure ZnO nanocrystals exhibited a strong and predominantly UV emission peaking at 377nm. In the photoluminescence spectra of mixed ZnO:Mn nanocrystals the major UV emission shifts from 377 to 408nm, and a strong blue emission appears at 435nm. The former is mainly induced by the impurity levels of Mn introduced in the band gap of the ZnO nanocrystals, while the latter is closely related to defect and Mn 2+ ions. With increasing Mn concentration the blue emission is enhanced due to the strong exchange interaction in the short range spin system and the excess impurities on the surface. The results show that the optical properties of ZnO can be tuned by the doping concentration of Mn. Mn doped ZnO nanocrystals with strong blue emission can be used in the fabrication of blue light devices.