A Multi-Object Spectrograph (MOS) based on micro-opto- electro-mechanical systems is one of the three core instruments selected for the NGST. Our group is involved in the preliminary studies on a promising solution for this instrument using a Micro-Mirror Array (MMA). We have focused our work towards three main topics: surface characterization of the micro-mirrors, MMA optical modeling, and optical design for the MOS. An accurate surface characterization method, based upon Foucault's knife-edge test has been developed, for measuring sub-nanometer deformations. Using the non-sequential ray tracing ability of our ray-tracing program, we have simulated a block of nine micro-mirrors with individual tilt angles, for properly designing an MMA- MOS. Finally, two different concepts for the MOS have been studied: a spectrograph with focal reduction, and a unit- magnification spectrograph preceded by a focal adaptator. The unit-magnification all-reflecting spectrograph is very promising, with a compact design, a perfectly plane image surface, and geometrical spots smaller than the detector pixels.