Willemite green phosphor powders have been prepared by metallo -organic decomposition(MOD) method and the photoluminescence and crystalline properties were studied as a function of both the firing temperature(800℃∼1100℃) and the concentration of Mn activator(4mol% ∼12mol%). Under 254nm excitation sources, the emission intensity of the phosphors was increased with increasing firing temperatures from 800℃ to 1000℃. From the XRD analysis, the powders heat treated above 1000℃ showed Willemite crystal structure. The maximum emission intensity was obtained for the phosphors with 8 mol% of Mn content heat treated at 1000℃. The concentration quenching was occurred at the Mn concentration above 10mol%. The phosphor particles showed almost spherical shapes with the average size of around 0.4∼0.5㎛ by the SEM morphology.