  In this paper, we study the null controllability of nonlinear wave systems.
Firstly, by combing Gerlerkin method and a fixed point theorem, we give the
null controllability for semi-linear wave equation with nonlinear function
dependents on velocities under the geometric control condition. Secondly, based
on a new iterative method, we prove the null controllability for a class of
quasi-linear wave system constructively. Finally, we give a control result for
a class of fully nonlinear wave system as application.
