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This study applies energy method to derive the system modeling of a tri-axial micro-accelerometer that consists of a quadri-beam suspension, a seismic mass, and displacement transducers using piezoelectric thin films. Two suspension beams support both ends of the seismic mass, which is fabricated by anisotropic etching of silicon. An out-of-plane acceleration will result in a symmetric bend, and in-plane accelerations will produce asymmetric bend and torsion of the suspension beams. Two piezoelectric thin-film transducers are arranged at both ends of each suspension and interconnected such that tri-axial accelerations can be measured selectively. The structure stiffness is derived from four laminated composite beams comprised of silicon substrates and piezoelectric films. The formulations of resonant frequencies and sensor sensitivities to tri-axial acceleration are presented and verified with finite element analysis.