Abstract: Considering the complex deformation of all flexible structure units, a stiffness analysis method of planar compliant mechanism is put forward. By treating each structure unit other than the base as the flexible unit, the whole structure stiffness can be obtained based on force equilibrium equations. Deformation of all the flexible units is synthetically considered in each direction, which makes the result close to its actual situation to a great extent. At the same time, a large amount of computational work is reduced by avoiding deformation-compatibility problem between flexible and rigid units, which must be dealt with separately for other methods. Stiffness of a micro-driving displacement amplifier module is analyzed by a few different methods.Results of the complete flexible unit method are much more close to that of ANSYS. The mechanism stiffness K is proved to be related to the ratio of power arm and resisting arm and gains minimum when the two arms are equal. The inflection point of stiffness-resisting arm history occurs just in this situation. These results are of great importance for the parameter design of lever-amplifier mechanism with given stiffness.