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This paper presents a method to realize complicated 3D microstructures for MEMS applications by multi-step SU-8 thick film preparation and X-ray lithography. SU-8 photoresist powder with solvent content of 7% is prepared by spin coating of standard SU-8 on a polymide sheet laminated aluminum plate. followed by baking and grinding. In reflow casting, a specific amount of SU-8 powder is loaded into a container formed by a square PDMS frame attached on a glass substrate. After heating, SU-8 powder melts and reflow resulting in a SU-8 thick film of a specific thickness. With sequential reflow casting and X-ray lithography, multilayer 3D microstructures of SU-8 can be obtained. A mathematical model is developed to find a suitable amount of SU-8 powder needed for powder casting of each consecutive layer based on SU-8 density, size of the casting frame and areas occupied by microstructures form previous layer. Fabrication of complicated three-layer 3D microstructures is demonstrated. The method can be used to fabricate all polymer MEMS devices as well as sacrificial micromold of microparts.