An improved delay-dependent robust stability criterion for power system with uncertain time-delay is presented. In the method， a Lyapunov function is constructed based on Lyapunov-Krasovskii theory， and the derivative function is then expressed as a set of linear matrix inequalities（LMI） which can be solved by the LMI toolboxes. Some necessary slack variables are introduced to its derivative function so as to reduce its conservativeness. The presented method is finally validated on a single-machine-infinite-bus system and the WSCC 3-generator-9-bus system. Cases of one time-delay and two time-delays with uncertainties are discussed. Numerical studies reveal that the presented method is correct and effective.