Background The nonlinear dynamic behavior of miniature stack-type piezoelectric actuators is not yet fully investigated and is still an open research field. Objective The objective of this paper is to determine an accurate dynamical model of an amplified stack-type piezoelectric actuator by using a recently developed nonlinear system identification method, namely Response-Controlled stepped-sine Testing (RCT). Method The nonlinear modal identification of a miniature stack-type piezoelectric actuator combined with a rhombus-type compliant mechanism is accomplished by using the RCT method. Several important modifications in the implementation of the RCT method imposed by the miniature nature of the piezo-actuator are successfully achieved for the first time in this study. Results Preliminary constant-voltage tests indicate strong softening nonlinearity with jump phenomenon at high voltage levels. In the nonlinear mode of interest, the RCT method quantifies about a 130 Hz change of natural frequency which corresponds to a 3% frequency shift, and a nonlinear modal damping ratio ranging from 1% to 1.5%, corresponding to a 50% change in the amplitude range of interest. Conclusion The validity of the single nonlinear mode theory on stack-type piezo-actuators and the quantification of the nonlinear modal damping of this type of actuators are achieved for the first time in this study to the best of the authors' knowledge. Consequently, an accurate nonlinear modal model is constructed which may help to estimate the parameters of a physical-driven (constitutive) model and therefore gain a better theoretical understanding of the nonlinear behavior of stack-type piezo-actuators. Furthermore, it is shown that the RCT method can be successfully applied for the modal identification of nonlinear miniature electro-mechanical systems.