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A dynamic model for a tri-axial micro-accelerometer with piezoelectric thin film sensing is presented using Lagrange's equation. A representative structure is used throughout instead of the more usual spring-mass type simplified model. The elastic properties of both the substrate and PZT thin film are included by use of laminated plate theory. The three out-of-plane bending motions of the accelerometer: symmetric, anti-symmetric and torsional are analysed. The accuracy of the dynamic model is confirmed by FEA. The dependence of structural parameters on the characteristics of the accelerometer for two end supported structures is discussed. The results show that the model gives close insights into the structural design of the tri-axial microaccelerometer and will be a useful tool for the design, analysis, optimisation, and characterization of a range of microaccelerometer devices, especially with regard to parameter optimisation and a trade-off between sensitivity and resonant frequency.