The inertia constant, a key parameter that makes power systems robust to disturbances and disruptions, has today been changed by factors such as the influence of renewable generation units and the reduction of traditional synchronous generators. Changes in the inertia of a power network play an important role in its performance, and a lack of knowledge of its status will have severe consequences for the network's online protection and control systems. Hence, knowledge of its online changes is essential for transmission system operators. This paper presents an online inertia estimation method that is based on inter-area electromechanical oscillations and simultaneously estimates the network inertia constant and the system damping coefficient. Unlike the previous methods that have used the flow of active power between two areas, the oscillation parameters are extracted from the voltage angle of the buses, improving the accuracy of the estimate. For this purpose and based on the classical swing equation, the relationship between the inertia constant of the system and the electromechanical oscillation parameters is first determined by the Prony technique. Then, the network equivalent inertia and damping coefficient are estimated. The efficiency and accuracy of the proposed method are verified by DigiSilent and MATLAB software and in several scenarios implemented on the IEEE 39-bus standard network. The simulation results show the appropriate accuracy of the proposed method. It is shown that the results of estimating the inertia and damping coefficient based on the active power signal have an average error percentage of 10 and 17.32, respectively. After using the voltage angle signal, the results are improved to 9.03 and 11.94, respectively. Due to the simplicity and small burden of calculations, the presented method can be easily used by network operators.