Mechanical amplifiers are employed to enhance output displacement of piezoelectric actuators. Dynamic behavior of such an amplified piezoelectric actuator needs to be known accurately in order to verify its suitability for the driven system. The objective of this study is to present a theoretical approach to determine a mathematical model for an amplified piezoelectric actuator (APA) which consists of a stacked piezoelectric actuator (SPA) with rhombus type mechanical amplifier (RPA). Dynamics of the mechanical amplifier is formulated based on distributed-parameter system approach, and Hamilton's principle is used to obtain a reduced order model. The SPA is then dynamically coupled with the reduced order model of a flexural amplifier by employing the constitutive relations between two substructures. The responses of the coupled system are calculated using linear vibration analysis under harmonic voltage input. Finally, the validity of the developed mathematical model is verified by comparing the calculated velocities with that of finite element solutions and experimental measurements in the frequency domain. The finite element (FE) solution is obtained using ANSYS software. Output velocity of the sample RPA is measured with the aid of a laser vibrometer. It is observed that the results obtained from the mathematical model show a very good agreement with those of the finite element analysis and test measurements.