Collimator is a crucial factor for MicroSPECT, which determines its spatial resolution, sensitivity, and detection-efficiency. Based on coded aperture theory, a new coded aperture collimator is designed; furthermore, its parameters and object-to-detector distance are optimized via Monte-Carlo simulation. MicroSPECT with this collimator is designed to achieve sub-millimeter geometric resolution, whose sensitivity will be more than 100 times higher than A-SPECT, and more than 10 times higher than ordinary Multipinhole-SPECT