This paper proposes a new and simple method based on Pade approximation and Routh-Hurwitz criterion to calculaté communication delay margin for time delayed automatic generation control (AGC) system. In modern power systems, due to extensive use of open and distributed channel communication the stability is a concern as these types of communication systems lead to instability. Although various complex algorithms can be used to compute the delay margin (critical delay), this paper proposes a simpler method which is based on Padé approximation. The accuracy of the method is presented in the results obtained with MATLAB / Simulink simulation. © 2019 IEEE.