GaAs-based micro-electro-mechanical systems (MEMS) are emerging technologies, which will lead to integration of new sensing and actuating devices with microwave and millimeter-wave electronics. However, developing the capability of selective deep etching of GaAs is a critical issue to be solved. This paper presents a study of a highly anisotropic deep GaAs etching. Two different plasma etching methods were studied for GaAs etching with chlorine-based chemistries: reactive ion etching (RIE) with Cl2/BCl3/SiCl4mixtures and high-density inductive coupled plasma (HDICP) etching with Cl2/BCl3 mixtures. A negative acting photoresist, SU-8, which allows high resolution, high aspect ratio patterning with strong resistance to etchants, was selected as the etching mask for this work. Process parameters were varied for best results, and the effects of the process parameters are presented. Issues such as etch selectivity of SU-8 and GaAs, reflow of SU-8, and loading effects were also investigated. An optimized etching process was developed to achieve a deep etch in excess of 300μm of GaAs with a single layer of SU-8 photoresist.