Gao, L. JACoW Publishing Geneva, Switzerland X-ray crystal monochromators of high thermal load exposed to white-beam in third generation synchrotron light sources are subject to thermal deformations that must be minimized using an optimized cooling structure. Finite-element analysis is used to calculate the crystal thermal deformations and optimize the cooling structure by changing crystal size and cooling structure. Optimization results are displayed. The heat absorbers consist of two copper cooling clamped to the crystal side faces via an indium foil in order to ensure a good thermal contact and to relax the thermal deformation. Those absorbers are each composed of 16 cooling fins regularly spaced and carved from the cooper block. The cooling fluid of high fin position is higher than the silicon crystal surface. The cooling structure can dissipate a heat load up to 800W& power density 10W/mm2.