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A novel model of a load-deflection method to determine the mechanical properties of micromembranes with compressive residual stress is described. Since thin film structures are frequently used in micro devices characterisation of mechanical properties of thin films is desired by the design and fabrication of micromachines. In this paper the mechanical properties of thin micromembranes under compressive stress are characterised which are fabricated by bulk micromachining. The relation between the center deflection and the load pressure on a square membrane is deduced by modelling the membrane as an elastic plate having large deflection with clamped boundaries. According to the model whether the membrane has initial deflection or not has no effect on the measurement result. The Young s modulus and residual stress are simultaneously determined. The mechanical properties of siliconoxide silicon nitride membranes and composite membranes of polysilicon with silicon nitride are measured.