This paper is concerned with the finite-time stability (FTS) of nonautonomous nonlinear systems. Firstly, a new definition of FTS for nonautonomous nonlinear systems is introduced. Then some sufficient conditions on FTS for nonautonomous nonlinear systems are established based on Lyapunov function. Furthermore, the conditions are shown to be necessary and sufficient for linear time-varying systems. Finally, several numerical examples are given to illustrate our results.