We present a high-quality UV-lithography process for making high aspect ratio microstructures for microengines using an ultrathick SU-8 photoresist layer. The microreciprocating engine project, which is ongoing at the University of Birmingham, aims to develop a compact power plant to replace batteries. The design of the microengine imposes strict requirements on the geometry of the engine components. The microfabrication research work has been concentrated on developing a SU-8 UV-lithography process to achieve good sidewall angles and high aspect ratio. Based on the study of the photoactive property of ultrathick SU-8 layers, an optimized prebake time is found for obtaining the minimum UV absorption of SU-8. The optimization process is tested and proven effective using a series of UV-lithography experiments on different prebake times. Microstructures with aspect ratios as high as 40:1 are produced in 1000-µm ultrathick SU-8 layers using standard UV-lithography equipment. The sidewall angles are controlled between 85 to 90 deg. The engine components fabricated using this process satisfy microengine design requirements.