In recent years, a relatively new type of negative photoresist, EPON SU-8, has received a lot of attention in the MEMS field because of its excellent lithography properties. Significant research efforts have been made to study the lithographic properties of SU-8 to obtain high aspect ratio microstructures with good sidewall quality. Currently, selection of optimal wavelengths of the UV light for lithographic and reduction of the diffraction effects are believed to be the two most important factors for achieving high-quality lithography of SU-8 as reported in the literature. Other reported efforts also include modifications of the chemical properties of SU-8 for better lithographic quality. We report a study on stress reduction during the postbaking process and the effects on lithography of ultra-thick high aspect ratio SU-8 microstructures. Our research proves that aspect ratios up to 40:1 in isolated open field structures of thicknesses between 1 and 1.5 mm can be obtained without any modifications of the resist chemistry or changes in light spectrum applied from a standard broadband UV source. The principal factor in this achievement is the reduction of internal stress during the postexposure bake process that eliminates large plastic deformations present during standard bake procedures. This process may be used for the fabrication of ultra-thick high aspect ratio microstructures that have to date only been obtainable using x-ray lithography-based LIGA processes. © 2004 Society of Photo-Optical Instrumentation Engineers.