Practical delay margin（PDM） is defined as the maximum time delay that system satisfies the following two constraints: real parts of all the eigenvalues are smaller than given values， and all the damping factors are larger than given values. A simple approach to determine PDM is presented. Critical eigenvalue and the corresponding PDM are determined by tracing eigenvalue locus of a complex matrix in a finite interval. Compared with linear matrix inequality（LMI） approach， the proposed method can identify the exact delay margin without any conservativeness and with a small computation burden. Single-machine infinite-bus and WSCC 3-generator-9-bus systems are employed to validate the effectiveness of the proposed method.