This paper reports a micro electro-mechanical system (MEMS) based sensor array integrated with CMOS-based optical readout. The integrated architecture has several unique features. MEMS devices are passive and there are no electrical connections to the MEMS sensor array. Thus, the architecture is scalable to large array formats for parallel measurement applications and can even be made as a disposable cartridge in the future using self-aligning features. A CMOS-based readout integrated circuit (ROIC) is integrated to the MEMS chip. Via holes are defined on ROIC by customized post-processing and MEMS chip is thinned down by a grinding process to enable integrated optical readout. A diffraction grating interferometer-based optical readout is realized by pixel-level illumination of the MEMS chip through the via holes and by capturing the reflected light using a photodetector array on the CMOS chip. A model for the optical readout principle has been developed using Fourier optics.