3D integration has been widely recognized as the next generation of technology for integrated microsystems with small form factor, high bandwidth, low power consumption, and possibility of heterogeneous More-than-Moore integration. Heterogeneous integration of MEMS and CMOS is critical in future development of multi-sensor data fusion in a low-cost chip size system. MEMS/CMOS integration was primarily done using monolithic and hybrid/package approaches until recently. In this work, 3D CMOS-on-MEMS stacking without TSV using direct (i.e. solder-less) metal bonding is demonstrated. This MEMS/CMOS integration leads to a simultaneous formation of electrical, mechanical, and hermetic bonds, eliminates chip-to-chip wire-bonding, and hence presents competitive advantages over hybrid or monolithic solutions. We present the fabrication flow and verify the performance of the stacked MEMS/CMOS microsystem in this paper.