There exist delays in the data transferring in power system PMU/WAMS（phasor measurement unit/wide area measurement system）. The estimation of maximum allowed delay time that will not cause the loss of system small signal stability is important in the design of a wide-area measurement based controller. Hence in this paper， the maximum allowed delay time is defined to be the delay stability margin and a method to determine the delay stability margin is proposed. Firstly， the transcendental characteristic equation of the time-delay system is converted into a polynomial equation based on Rekasius transformation. Two equations before and after Rekasius transformation are identical when there is a system eigenvalue on the imaginary axis. Secondly， Routh stability criterion is used to determine the critical eigenvalue， the critical oscillation frequency and the delay stability margin. Finally， a single-machine infinite-bus system is used to demonstrate the proposed method. It is found that when the generation output and exciter gain increase， the delay stability margin decreases. While the impact of the generator damping to the delay stability margin is very complicated that needs further studies in future.