The CsI(Tl) Detector Element (CDE) is the unit of detection of the calorimeter subsystem of the Large Area Telescope (LAT) of the GLAST satellite (IEEE Nuclear Science Symposium, 2002), which will be launched in September 2006. The LAT consists of 4×4 identical towers, each one including a strip silicon tracker on the top and a CsI(Tl) calorimeter on the bottom. A segmented plastic scintillator anticoincidence detector surrounds the 16 towers. The calorimeter modules, 8 layers of 12 CDEs each, have a hodoscope's shape to allow the measurement of the gamma-ray direction. The CDE main characteristics are its Light Yield, the energy resolution and the tapering, ratio of left/right signals used for the determination of the crossing position of the particle in the crystal. The CDE have to operate in space at about −10°C. We will describe the CDE, the optimization of its main parameter, the L.Y., and finally its variation with temperature.