The characteristics of fluorescent plates for high energy electron beams (0.5-2.0 MeV) are examined. The thickness and the optical transparency of plates strongly affect the luminous broadening and intensity. The spatial luminous broadening in fluorescent plates is measured and is simply represented by the rise width of a knife edge image. When the thickness is much smaller than the range of incident electrons, the rise width is 1/4-1/5 of the thickness in the case of YAG single crystal plates that are transparent for light, while the rise width is nearly equal to the thickness for the packed P22 powder plates that are opaque for light. To suppress the luminous broadening under 50 microm, the thickness of YAG plates has to be thinner than 250 microm in the energy region around 2 MeV. Under the same condition of the rise width, the luminous intensity of YAG plates is twice as high as that of the P22 plates.