Low-dimensional nanosized semiconductor materials, such as quantum dots, have attracted considerable interest in the past decade. Due to their size-dependent physical and optical properties, these nanosized semiconductor materials offer various technological applications in photoelectronic devices. Among the II-VI semiconductor materials, zinc selenide (ZnSe) is of special interest due to its intense UV blue light emission at 460 nm wavelength with a band gap of 2.7 eV, which has not been observed in other well-known semiconductor materials, such as CdS and CdSe. In addition, it has been reported that when the surface of ZnSe is passivated by another semiconductor nanocrystal layer such as ZnS, a ZnSe/ZnS core shell quantum dot can be formed, and the quantum yield of the ZnSe/ZnS quantum dot is much higher (about 20 times) than that of bare ZnSe nanocrystallite. The preparation method of ZnSe or ZnSe/ZnS quantum dot often includes a thermal decomposition reaction of organometallic precursors in a hot coordinating organic solvent such as trioctylphosphineoxide (TOPO), leading to a hydrophobic surface on the resulting nanocrystal. Previously, in this lab, water-dispersible ZnSe and ZnSe/ZnS nanocrystals were also prepared by exchanging the hydrophobic nanocrystal surface with polar organic ligands such as mercaptoacetic acid (MAA) and ethylenediaminetetraacetic acid (EDTA) molecules. Manganese-ion doped ZnSe nanocrystals have been prepared in various media. In most cases, emission wavelengths of the ZnSe:Mn nanocrystals were far shifted from that of the undoped ZnSe nanocrystal. They usually emit orangecolored lights (580-600 nm) due to the dopant Mn ions. In this paper we report on the synthesis of Mn ion-doped ZnSe nanocrystal via thermal decomposition reaction from organometallic precursors, and unexpected white light emission from the prepared ZnSe:Mn nanocrystal. Figure 1 presents an HR-TEM image of ZnSe:Mn nanocrystal. In the picture, the measured and calculated average particle size was 3.5 nm. In addition, the appearance of distinct lattice planes in the fringe image with an approximate spacing of 3.4 A suggests that all the solid samples were made of single crystals rather than poly-crystalline aggregate mixtures. An energy dispersive X-ray spectroscopy diagram (EDXS, in Fig. 2) was also obtained to confirm the elemental compositions of the ZnSe:Mn nanocrystal in the solid state. The obtained doping concentration of manganese(II) ions in the ZnSe:Mn nanocrystal was 2.3 atomic %. To determine the doping concentration of Mn ions more precisely, Inductively Coupled Plasma-Atomic Emission Spectrometry (ICPAES) analysis was performed. Three trials of the sample measurements revealed that the average elemental proportion of the Mn ions relative to ZnSe parent crystal was Figure 1. HR-TEM image of ZnSe:Mn nanocrystal, the scale bar represents 5 nm.