This talk introduces some novel delay differential inequalities with time-varying coefficients, and recasts the synchronization problems of multi-agent systems with nonlinear nodal dynamics into the stabilization of delayed differential equation. With the aid of the proposed differential equalities, some sufficient criteria for reaching synchronization of multi-agent systems with time-varying control are thus established. Some intermittent control policies are also studied under the aforementioned framework. It is of interest to observe that the time average of the control strength is decisive to synchronization. The largest admissible delay can also be estimated. Numerical examples are also given to demonstrate and verify the theoretical results.