This paper reports a new approach for design and fabrication of chip-level inertial measurement units (IMUs). The method utilizes a 3-D foldable silicon-on-insulator (SOI) backbone with in-situ fabricated high-aspect-ratio sensors. A planar multisensor unit was fabricated and subsequently folded in a pyramidal shape, forming a compact IMU. Inertial characterization of the sensors integrated on the IMU pyramid was performed at atmospheric pressure. Structural rigidity and sensor axis alignment stability of the folded IMUs have been characterized under various environmental conditions, including vibration, thermal loading, thermal shock, and constant acceleration. The maximum angular misalignment due to variation in environmental conditions between IMU pyramid sidewalls was shown to be less than 4 and 0.2 mrad for epoxy and solder reinforced structures, respectively. Vibration testing revealed no resonances up to 10 kHz in the assembled 3-D structures. Our results confirm feasibility of the fabrication approach.