Abstract: A method is proposed in this paper to investigate the oscillatory stability using the logarithmic derivative of aggregated impedance determinant. It can be applied to system with "black/gray box" models and can quantify the damping and frequency of the dominant oscillation modes. The method first obtains the aggregated impedance (matrix) through the impedance network model, then computes the logarithmic derivative of its determinant in the frequency domain, and finally finds the zero of the aggregated impedance determinant to quantify the stability of system oscillation according to the real and imaginary curves of the logarithmic derivative. The method is applied to a grid-connected voltage source converter system and a large wind power system, respectively. Its accuracy and efficiency are verified by comparing the performance with the MATLAB function.